aarch64: Fix ls64 intrinsic availability

The availability of ls64 intrinsics and data types were determined
solely by the globally specified architecture features, which did not
reflect any changes specified in target pragmas or attributes.

This patch removes the initialisation-time guards for the intrinsics,
and replaces them with checks at use time. We also get better error
messages when ls64 is not available (matching the existing error
messages for SVE intrinsics).

The data512_t type is made always available; this is consistent with the
present behaviour for Neon fp16/bf16 types.

gcc/ChangeLog:

	PR target/112108
	* config/aarch64/aarch64-builtins.cc (handle_arm_acle_h): Remove
	feature check at initialisation.
	(aarch64_general_check_builtin_call): Check ls64 intrinsics.
	* config/aarch64/arm_acle.h: (data512_t) Make always available.

gcc/testsuite/ChangeLog:

	PR target/112108
	* gcc.target/aarch64/acle/ls64_guard-1.c: New test.
	* gcc.target/aarch64/acle/ls64_guard-2.c: New test.
	* gcc.target/aarch64/acle/ls64_guard-3.c: New test.
	* gcc.target/aarch64/acle/ls64_guard-4.c: New test.