Support a seL4 aarch64 build

aarch64: fix CRTJMP in reloc.h

commit f3ddd173806fd5c60b3f034528ca24542aecc5b9 broke the build by
using "bx" instead of "br".

dynamic linker bootstrap overhaul

this overhaul further reduces the amount of arch-specific code needed
by the dynamic linker and removes a number of assumptions, including:

- that symbolic function references inside libc are bound at link time
via the linker option -Bsymbolic-functions.

- that libc functions used by the dynamic linker do not require
access to data symbols.

- that static/internal function calls and data accesses can be made
without performing any relocations, or that arch-specific startup
code handled any such relocations needed.

removing these assumptions paves the way for allowing libc.so itself
to be built with stack protector (among other things), and is achieved
by a three-stage bootstrap process:

1. relative relocations are processed with a flat function.
2. symbolic relocations are processed with no external calls/data.
3. main program and dependency libs are processed with a
fully-functional libc/ldso.

reduction in arch-specific code is achived through the following:

- crt_arch.h, used for generating crt1.o, now provides the entry point
for the dynamic linker too.

- asm is no longer responsible for skipping the beginning of argv[]
when ldso is invoked as a command.

- the functionality previously provided by __reloc_self for heavily
GOT-dependent RISC archs is now the arch-agnostic stage-1.

- arch-specific relocation type codes are mapped directly as macros
rather than via an inline translation function/switch statement.

add aarch64 port

This adds complete aarch64 target support including bigendian subarch.

Some of the long double math functions are known to be broken otherwise
interfaces should be fully functional, but at this point consider this
port experimental.

Initial work on this port was done by Sireesh Tripurari and Kevin Bortis.