feat(gcs): support guarded control stack

feat(gcs): support guarded control stack

Arm v9.4 introduces support for Guarded Control Stack, providing
mitigations against some forms of RPO attacks and an efficient mechanism
for obtaining the current call stack without requiring a full stack
unwind. Enable access to this feature for EL2 and below, context
switching the newly added EL2 registers as appropriate.

Change the FVP platform to default to handling this as a dynamic option
so the right decision can be made by the code at runtime.

Signed-off-by: Mark Brown <broonie@kernel.org>
Change-Id: I691aa7c22e3547bb3abe98d96993baf18c5f0e7b