posix arch: build on aarch64 / allow host-specific cmake includes

This change enables specific compiler and linker options to be used in
the case that an arch/posix/os.arch.cmake file exists.

Note: os and arch in the above case are evaluations of
CMAKE_HOST_SYSTEM_NAME and CMAKE_HOST_SYSTEM_PROCESSOR.

Otherwise, the existing "generic" compiler and linker flags in
arch/posix/CMakeLists.txt are used.

Additional flags and checks are provided in
arch/posix/Linux.aarch64.cmake.

Added scripts/user_wordsize.py to detect if userspace is 64-bit or
32-bit, which should be consistent with the value of CONFIG_64BIT
for Aarch64 on Linux.

Fixes #24842

Signed-off-by: Christopher Friedt <chrisfriedt@gmail.com>