aarch64: Use RTL builtins for vmull[_high]_p8 intrinsics

aarch64: Use RTL builtins for vmull[_high]_p8 intrinsics

Rewrite vmull[_high]_p8 Neon intrinsics to use RTL builtins rather
than inline assembly code, allowing for better scheduling and
optimization.

gcc/ChangeLog:

2021-02-05  Jonathan Wright  <joanthan.wright@arm.com>

	* config/aarch64/aarch64-simd-builtins.def: Add pmull[2]
	builtin generator macros.
	* config/aarch64/aarch64-simd.md (aarch64_pmullv8qi): Define.
	(aarch64_pmull_hiv16qi_insn): Define.
	(aarch64_pmull_hiv16qi): Define.
	* config/aarch64/arm_neon.h (vmull_high_p8): Use RTL builtin
	instead of inline asm.
	(vmull_p8): Likewise.