aarch64: Add GCS builtins

aarch64: Add GCS builtins

Add new builtins for GCS:

  void *__builtin_aarch64_gcspr (void)
  uint64_t __builtin_aarch64_gcspopm (void)
  void *__builtin_aarch64_gcsss (void *)

The builtins are always enabled, but should be used behind runtime
checks in case the target does not support GCS. They are thin
wrappers around the corresponding instructions.

The GCS pointer is modelled with void * type (normal stores do not
work on GCS memory, but it is writable via the gcsss operation or
via GCSSTR if enabled so not const) and an entry on the GCS is
modelled with uint64_t (since it has fixed size and can be a token
that's not a pointer).

gcc/ChangeLog:

	* config/aarch64/aarch64-builtins.cc (enum aarch64_builtins): Add
	AARCH64_BUILTIN_GCSPR, AARCH64_BUILTIN_GCSPOPM, AARCH64_BUILTIN_GCSSS.
	(aarch64_init_gcs_builtins): New.
	(aarch64_general_init_builtins): Call aarch64_init_gcs_builtins.
	(aarch64_expand_gcs_builtin): New.
	(aarch64_general_expand_builtin): Call aarch64_expand_gcs_builtin.

Co-authored-by: Richard Sandiford <richard.sandiford@arm.com>