kselftest/arm64: Validate that GCS push and write permissions work

kselftest/arm64: Validate that GCS push and write permissions work

Add trivial assembly programs which give themselves the appropriate
permissions and then execute GCSPUSHM and GCSSTR, they will report errors
by generating signals on the non-permitted instructions. Not using libc
minimises the interaction with any policy set for the system but we skip on
failure to get the permissions in case the system is locked down to make
them inaccessible.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20241005-arm64-gcs-test-flags-v1-1-03cb9786c5cd@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>