aarch64: Add -mbranch-protection=gcs option

aarch64: Add -mbranch-protection=gcs option

This enables Guarded Control Stack (GCS) compatible code generation.

The "standard" branch-protection type enables it, and the default
depends on the compiler default.

gcc/ChangeLog:

	* config/aarch64/aarch64-protos.h (aarch_gcs_enabled): Declare.
	* config/aarch64/aarch64.cc (aarch_gcs_enabled): Define.
	(aarch_handle_no_branch_protection): Handle gcs.
	(aarch_handle_standard_branch_protection): Handle gcs.
	(aarch_handle_gcs_protection): New.
	* config/aarch64/aarch64.opt: Add aarch_enable_gcs.
	* configure: Regenerate.
	* configure.ac: Handle gcs in --enable-standard-branch-protection.
	* doc/invoke.texi: Document -mbranch-protection=gcs.