aarch64: Add +sme

aarch64: Add +sme

This patch adds the +sme ISA feature and requires it to be present
when compiling arm_streaming code.  (arm_streaming_compatible code
does not necessarily assume the presence of SME.  It just has to
work when SME is present and streaming mode is enabled.)

gcc/
	* doc/invoke.texi: Document SME.
	* doc/sourcebuild.texi: Document aarch64_sve.
	* config/aarch64/aarch64-option-extensions.def (sme): Define.
	* config/aarch64/aarch64.h (AARCH64_ISA_SME): New macro.
	(TARGET_SME): Likewise.
	* config/aarch64/aarch64.cc (aarch64_override_options_internal):
	Ensure that SME is present when compiling streaming code.

gcc/testsuite/
	* lib/target-supports.exp (check_effective_target_aarch64_sme): New
	target test.
	* gcc.target/aarch64/sme/aarch64-sme.exp: Force SME to be enabled
	if it isn't by default.
	* g++.target/aarch64/sme/aarch64-sme.exp: Likewise.
	* gcc.target/aarch64/sme/streaming_mode_3.c: New test.