aarch64: Add support for GCS system registers with the +gcs modifier

aarch64: Add support for GCS system registers with the +gcs modifier

Given the introduction of system registers associated with the Guarded
Control Stack extension to Armv9.4-a in Binutils and their reliance on
the `+gcs' modifier, we implement the necessary changes in GCC to
allow for them to be recognized by the compiler.

gcc/ChangeLog:

	* config/aarch64/aarch64-option-extensions.def (gcs): New.
	* config/aarch64/aarch64.h (AARCH64_ISA_GCS): New.
	(TARGET_THE):  Likewise.
	* doc/invoke.texi (AArch64 Options): Describe GCS.