aarch64: Use RTL builtins for integer mls_n intrinsics

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

gcc/ChangeLog:

2021-01-15  Jonathan Wright  <jonathan.wright@arm.com>

	* config/aarch64/aarch64-simd-builtins.def: Add mls_n builtin
	generator macro.
	* config/aarch64/aarch64-simd.md (*aarch64_mls_elt_merge<mode>):
	Rename to...
	(aarch64_mls_n<mode>): This.
	* config/aarch64/arm_neon.h (vmls_n_s16): Use RTL builtin
	instead of asm.
	(vmls_n_s32): Likewise.
	(vmls_n_u16): Likewise.
	(vmls_n_u32): Likewise.
	(vmlsq_n_s16): Likewise.
	(vmlsq_n_s32): Likewise.
	(vmlsq_n_u16): Likewise.
	(vmlsq_n_u32): Likewise.