Constructors between vsyscall init and libc init

Add a region for constructors to be placed between __sysinfo being
initialised to sel4_vsyscall and libc_init to allow a user-level
implementation to set up virtual memory syscalls use by libc_init whilst
depending on a functioning implementation of malloc.

Signed-off-by: Curtis Millar <curtis.millar@data61.csiro.au>

Force GCC to find new location of TLS base

GCC will sometimes optimise accesses to TLS across function calls
assuming that the TLS base address has not been changed and thus none of
the TLS variables have been relocated.

Given initialising musl libc explicitly creates a new TLS region,
updating __sel4_ipc_buffer requires us to force GCC to find its address
in the new TLS region rather than re-using the address it already
determined.

Signed-off-by: Curtis Millar <curtis.millar@data61.csiro.au>

Add config option to delay libc initialisation

The initialisation of libc should occur after system calls have been
initalised. Some calls are initialised externally or have external
dependencies.

Signed-off-by: Curtis Millar <curtis.millar@data61.csiro.au>

Use constructor to initialise libc

This initialises the C standard library using a constructor executed
from the sel4 runtime.

Signed-off-by: Curtis Millar <curtis.millar@data61.csiro.au>

libsel4muslcsys: Implement sys_write wrapper

This wraps sys_writev the same way that sys_read is implemented.

Trivial: Style fix

make the style checker happy

seL4_libs: remove autoconf.h from seL4_libs

this commit removes autoconf.h generation from each lib and
append '#include <<lib_name>/gen_config.h>' after each
'#include <autoconf.h>' since autoconf.h is only the for the
kernel config now. This is a temporarily solution, since we
currently don't have a way to tell which header files each file
is trying to include when it includes 'autoconf.h'

libsel4muslcsys: transition to new libcpio API

libsel4muslcsys: Avoid calling TCB_SetTLSBase unless needed

The C library only calls the set_thread_pointer syscall if it believes it cannot set
the thread pointer itself. This handles a specific kernel configuration on x86-64 where
we actually can set the thread pointer from user level without resorting to the TCB
invocation.

libsel4muslcsys: Call TCB_SetTLSBase when possible

This commit provides support for initial thread in a process, that calls __libc_start_main,
to get a correctly setup thread pointer. If the boot_tcb_cptr environment variable is
set either by the proccess creator, or in the case of the rootserver it will be constructed
by _sel4_start, then during libc's init process, prior to main being called, the
boot_sys_set_thread_area will attempt to set the thread pointer.

Should boot_tcb_cptr not be set an application could still use
muslcsys_get_boot_set_thread_area and then do the equivalent of the code in t his
commit, but this provides a much nicer approach.

sel4muslcsys: Alias __ARM_NR_set_tls to __NR_set_thread_area in boot functions

The ARM set_tls syscalls behaves effectively the same as the x86 set_thread_area syscall,
and the boot implementation provided currently can be used as a dual implementation.

As they are *not* actually the same syscall post boot the 'real' sys_set_thread_area is
not provided as the implementation of __ARM_NR_set_tls

sel4muslcsys: Implement sparse syscall table for ARM syscalls

Some ARM syscalls have quite high effective numbers so this adds a sparse syscall table
that is a lookup of syscall to handler, instead of the normal table that encodes the
syscall number in the table index. As there are only 5 syscalls this is currently
implemented as an unordered list that must be searched to find any entry

libsel4muslcsys: Early debug output prior to C library init

The generic sys_writev implementation most likely will crash/not work if it gets
called prior to the C library having been initialized (i.e. at least prior to the init
array functions getting called). The idea behind this change is to explicitly just
support output via seL4_DebugPutChar (if it exists) prior to C library initialization,
and then switch to the actual sys_writev.

Whilst the boot_sys_writev should never be used, it makes it possible to debug very
early boot code, especially in a rootserver that may need to do non trivial startup
logic in its entry point before calling the muslc entry point.

Fix licenses

libsel4muslcsys: Provide run time CPIO file system support

This changes the existing build time config support for a CPIO file system into a run
time option. The build time configuration option is still used to decide whether to
use the new interface to populate with the default cpio archive symbols

SELFOUR-862: CONFIG_PRINTING --> seL4_DebugPutChar

trivial: fix warnings

libsel4muslcsys: Allow syscalls to be overriden at run time

Adds an interface to override syscalls at run time. For syscalls that happen
before a run time handler can be installed an interface is also provided for
querying the attempted invocation.

libsel4muslcsys: Add support for openat syscall

libsel4muslcsys: Implement mmap syscall

Changes the current mmap2 implementation to be an mmap implementation,
and redirects the mmap2 syscall to mmap

libsel4muslcsys: create dummy implementation of sys_madvise()

libsel4muslcsys: Skip syscalls that are only defined on 32-bit systems if they are not defined

libsel4muslcsys: Add some missing thread routines

A C library may call set_thread_area on startup and tkill on shutdown,
so we need to implement stubs of these

libsel4muslcsys: Fix missing #include.

libsel4muslcsys: Use libutils' compiler attribute abstractions.

Depends on https://github.inside.nicta.com.au/seL4/util_libs/pull/10.