aarch64: Use RTL builtins for [su]mull_high_n intrinsics

aarch64: Use RTL builtins for [su]mull_high_n intrinsics

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

gcc/ChangeLog:

2021-02-03  Jonathan Wright  <jonathan.wright@arm.com>

	* config/aarch64/aarch64-simd-builtins.def: Add [su]mull_hi_n
	builtin generator macros.
	* config/aarch64/aarch64-simd.md
	(aarch64_<su>mull_hi_n<mode>_insn): Define.
	(aarch64_<su>mull_hi_n<mode>): Define.
	* config/aarch64/arm_neon.h (vmull_high_n_s16): Use RTL builtin
	instead of inline asm.
	(vmull_high_n_s32): Likewise.
	(vmull_high_n_u16): Likewise.
	(vmull_high_n_u32): Likewise.