arm64/gcs: Add manual encodings of GCS instructions

Define C callable functions for GCS instructions used by the kernel. In
order to avoid ambitious toolchain requirements for GCS support these are
manually encoded, this means we have fixed register numbers which will be
a bit limiting for the compiler but none of these should be used in
sufficiently fast paths for this to be a problem.

Note that GCSSTTR is used to store to EL0.

Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20241001-arm64-gcs-v13-9-222b78d87eee@kernel.org


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