AArch64 GNU example build

Builds the ThreadX library and a sample image for the AArch64 port this directory belongs to.

./build_threadx.sh          # the ThreadX library, tx.a
./build_threadx_sample.sh   # the sample image, sample_threadx.out

Set TOOLCHAIN=atfe to build with Arm Toolchain for Embedded, which is LLVM based, instead of the GNU toolchain:

TOOLCHAIN=atfe ATFE_CLANG=/path/to/clang ./build_threadx.sh
TOOLCHAIN=atfe ATFE_CLANG=/path/to/clang ./build_threadx_sample.sh

Both scripts derive the -mcpu value and the kernel source directory from the port directory they sit in, so one pair of scripts serves every AArch64 port, SMP included. Before they existed these examples could only be built through the Arm Development Studio project files beside them, which is why the .cproject and .launch files remain.

What the sample targets

A fixed virtual platform: a GICv3 interrupt controller and an SP804 timer at the Base platform addresses, as set up by startup.S, vectors.S and the GICv3 sources here. It will not run on a model or board that places those peripherals elsewhere without changing the addresses.

Semihosting

startup.S calls initialise_monitor_handles to open the standard file handles over a debugger connection. The GNU toolchain provides it in libgloss, pulled in by --specs=rdimon.specs. picolibc, which Arm Toolchain for Embedded ships, has no equivalent and neither does its semihosting library, so semihost_stub.S supplies a weak no-op for that toolchain only. A real definition always wins over it.