AArch64: Add SME LUTv2 architecture extension

AArch64: Add SME LUTv2 architecture extension

Add the SME LUTv2 architecture extension. Users can enable the extension
by adding `+sme-lutv2` to `-march` or `-mcpu`, and test for its presence
with the `__ARM_FEATURE_SME_LUTv2` macro. The intrinsics will be added
in the next commit.

gcc/ChangeLog:

	* config/aarch64/aarch64-c.cc (aarch64_update_cpp_builtins): Conditonally define
	`__ARM_FEATURE_SME_LUTv2" macro.
	* config/aarch64/aarch64-option-extensions.def (AARCH64_OPT_EXTENSION("sme-lutv2")): New
	optional architecture extension.
	* config/aarch64/aarch64.h (TARGET_SME_LUTv2): New macro.
	* doc/invoke.texi: Document `+sme-lutv2` flag.