aarch64: Add support for __arm_locally_streaming

aarch64: Add support for __arm_locally_streaming

This patch adds support for the __arm_locally_streaming attribute,
which allows a function to use SME internally without changing
the function's ABI.  The attribute is valid but redundant for
__arm_streaming functions.

gcc/
	* config/aarch64/aarch64.cc (aarch64_arm_attribute_table): Add
	arm::locally_streaming.
	(aarch64_fndecl_is_locally_streaming): New function.
	(aarch64_fndecl_sm_state): Handle locally-streaming functions.
	(aarch64_cfun_enables_pstate_sm): New function.
	(aarch64_add_offset): Add an argument that specifies whether
	the streaming vector length should be used instead of the
	prevailing one.
	(aarch64_split_add_offset, aarch64_add_sp, aarch64_sub_sp): Likewise.
	(aarch64_allocate_and_probe_stack_space): Likewise.
	(aarch64_expand_mov_immediate): Update calls accordingly.
	(aarch64_need_old_pstate_sm): Return true for locally-streaming
	streaming-compatible functions.
	(aarch64_layout_frame): Force all call-preserved Z and P registers
	to be saved and restored if the function switches PSTATE.SM in the
	prologue.
	(aarch64_get_separate_components): Disable shrink-wrapping of
	such Z and P saves and restores.
	(aarch64_use_late_prologue_epilogue): New function.
	(aarch64_expand_prologue): Measure SVE lengths in the streaming
	vector length for locally-streaming functions, then emit code
	to enable streaming mode.
	(aarch64_expand_epilogue): Likewise in reverse.
	(TARGET_USE_LATE_PROLOGUE_EPILOGUE): Define.
	* config/aarch64/aarch64-c.cc (aarch64_define_unconditional_macros):
	Define __arm_locally_streaming.

gcc/testsuite/
	* gcc.target/aarch64/sme/locally_streaming_1.c: New test.
	* gcc.target/aarch64/sme/locally_streaming_2.c: Likewise.
	* gcc.target/aarch64/sme/locally_streaming_3.c: Likewise.
	* gcc.target/aarch64/sme/locally_streaming_4.c: Likewise.
	* gcc.target/aarch64/sme/keyword_macros_1.c: Add
	__arm_locally_streaming.
	* g++.target/aarch64/sme/keyword_macros_1.C: Likewise.