aarch64: Use RTL builtins for integer mls intrinsics

aarch64: Use RTL builtins for integer mls intrinsics

Rewrite integer mls Neon intrinsics to use RTL builtins rather than
inline assembly code, allowing for better scheduling and
optimization.

gcc/Changelog:

2021-01-11  Jonathan Wright  <jonathan.wright@arm.com>

	* config/aarch64/aarch64-simd-builtins.def: Add mls builtin
	generator macro.
	* config/aarch64/arm_neon.h (vmls_s8): Use RTL builtin rather
	than asm.
	(vmls_s16): Likewise.
	(vmls_s32): Likewise.
	(vmls_u8): Likewise.
	(vmls_u16): Likewise.
	(vmls_u32): Likewise.
	(vmlsq_s8): Likewise.
	(vmlsq_s16): Likewise.
	(vmlsq_s32): Likewise.
	(vmlsq_u8): Likewise.
	(vmlsq_u16): Likewise.
	(vmlsq_u32): Likewise.