kselftest/arm64: Add test coverage for GCS mode locking

Verify that we can lock individual GCS mode bits, that other modes
aren't affected and as a side effect also that every combination of
modes can be enabled.

Normally the inability to reenable GCS after disabling it would be an
issue with testing but fortunately the kselftest_harness runs each test
within a fork()ed child.  This can be inconvenient for some kinds of
testing but here it means that each test is in a separate thread and
therefore won't be affected by other tests in the suite.

Once we get toolchains with support for enabling GCS by default we will
need to take care to not do that in the build system but there are no
such toolchains yet so it is not yet an issue.

Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
Tested-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20241001-arm64-gcs-v13-36-222b78d87eee@kernel.org


Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>