Added build scripts for the AArch64 examples, which had none (#597)

The AArch64 examples could only be built through the Arm Development Studio
project files beside them. There was no script, so nothing in CI or on a
developer's machine could build them, and the sources they need were never
named anywhere: vectors.S, v8_aarch64.S and v8_utils.S were present but
unreferenced, which is why a hand written link failed on GetCPUID, GetAffinity,
InvalidateUDCaches and ZeroBlock. Those four are defined in v8_aarch64.S and
v8_utils.S, in the tree all along.

Add one pair of scripts, in ports_arch so every AArch64 port receives them.
Both derive the -mcpu value and the kernel source directory from the port
directory they sit in, so a single pair serves the twelve ThreadX ports and the
twelve SMP ports, the latter building against common_smp and picking up the C
sources those ports carry alongside the assembly. TOOLCHAIN=atfe selects Arm
Toolchain for Embedded in place of the GNU toolchain, as it does for the ARMv7
example scripts.

One symbol genuinely had no definition. startup.S calls
initialise_monitor_handles to open the standard file handles over a debugger
connection; the GNU toolchain provides it in libgloss through
--specs=rdimon.specs, while picolibc has no equivalent and neither does the
LLVM toolchain's semihosting library. semihost_stub.S supplies a weak no-op,
linked for that toolchain only, so a real definition always wins.

Extend scripts/check_clang.sh to cover ports_smp as well as ports, since the
example builds now exist there too.

Verified by building every AArch64 example with Arm Toolchain for Embedded:
twelve ThreadX ports at 314,744 to 315,256 bytes of text and twelve SMP ports
at 325,304 to 325,880. scripts/check_clang.sh now reports 35 of 35 example
builds linking with none failing, the twenty-four new ones plus the eleven that
already built. The images have not been executed: the sample
targets the Base platform peripheral addresses, which the available emulator
does not provide.

Cortex-A34, and the Cortex-A34 and A78 SMP ports, are left out. They are not in
the generator's core list, so they receive nothing from ports_arch and cannot be
checked for reproducibility either. Bringing them in rewrites 114 files in those
three directories, which deserves its own change.

Assisted-by: Claude Code (Opus 5) <[email protected]>

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