aarch64: Use RTL builtins for [su]mlsl_n intrinsics

aarch64: Use RTL builtins for [su]mlsl_n intrinsics

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

gcc/ChangeLog:

2021-01-27  Jonathan Wright  <jonathan.wright@arm.com>

	* config/aarch64/aarch64-simd-builtins.def: Add [su]mlsl_n
	builtin generator macros.
	* config/aarch64/aarch64-simd.md (aarch64_<su>mlsl_n<mode>):
	Define.
	* config/aarch64/arm_neon.h (vmlsl_n_s16): Use RTL builtin
	instead of inline asm.
	(vmlsl_n_s32): Likewise.
	(vmlsl_n_u16): Likewise.
	(vmlsl_n_u32): Likewise.