aarch64: Add support for fp8dot2 and fp8dot4

aarch64: Add support for fp8dot2 and fp8dot4

The AArch64 FEAT_FP8DOT2 and FEAT_FP8DOT4 extension introduces
instructions for dot product of vectors.

This patch introduces the following intrinsics:
1. vdot{q}_{fp16|fp32}_mf8_fpm.
2. vdot{q}_lane{q}_{fp16|fp32}_mf8_fpm.

We added a new aarch64_builtin_signature variant, ternary_lane, and added
support for it in the functions aarch64_fntype and
aarch64_expand_pragma_builtin.

gcc/ChangeLog:

	* config/aarch64/aarch64-builtins.cc
	(enum class): Add ternary_lane.
	(aarch64_fntype): Hnadle ternary_lane.
	(aarch64_pragma_builtins_checker::require_immediate_lane_index): New
	function.
	(aarch64_pragma_builtins_checker::check): Handle the new intrinsics.
	(aarch64_expand_pragma_builtin): Likewise.
	* config/aarch64/aarch64-c.cc
	(aarch64_update_cpp_builtins): Define TARGET_FP8DOT2 and
	TARGET_FP8DOT4.
	* config/aarch64/aarch64-simd-pragma-builtins.def: Define vdot
	and vdot_lane intrinsics.
	* config/aarch64/aarch64-simd.md
	(@aarch64_<fpm_uns_op><mode>): New pattern.
	(@aarch64_<fpm_uns_op>_lane<VQ_HSF_VDOT:mode><VB:mode>): Likewise.
	* config/aarch64/iterators.md (VQ_HSF_VDOT): New mode iterator.
	(UNSPEC_VDOT, UNSPEC_VDOT_LANE): New unspecs.
	(fpm_uns_op): Handle them.
	(VNARROWB, Vnbtype): New mode attributes.
	(FPM_VDOT, FPM_VDOT_LANE): New int iterators.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/pragma_cpp_predefs_4.c: Test fp8dot2 and fp8dot4.
	* gcc.target/aarch64/simd/vdot2_fpm.c: New test.
	* gcc.target/aarch64/simd/vdot4_fpm.c: New test.
	* gcc.target/aarch64/simd/vdot_lane_indices_1.c: New test.

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