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>

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>

trivial: style fix

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>

sel4muslcsys: Set -u __vsyscall_ptr on link args

Projects that use this library expect __vsyscall_ptr to be present in
the final binary, but require forcing the symbol because it is often not
used by the final binary but by the loader when setting up the AUX
vectors. Adding -u __vsyscall_ptr to the target_link_libraries property
of this library means that the right linker flags will be automatically
provided and applications don't need to keep providing this setting
themselves.

replace kernel_autoconf with sel4_autoconf

since the mainline kernel's kernel_autoconf doesn't include
seL4/gen_config.h but sel4_autoconf does

link kernel_autoconf explicitly

explicitly link kernel_autoconf on each subdirectory

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'

CMake: Mark config options as advanced

This is to hide the options in the cmake-gui. Top level applications can
choose to reexpose these options by clearing the advanced setting.

trivial: style CMakeLists.txt files

Don't add files that don't exist

Add CMake build rules for various libraries