aarch64: Use RTL builtins for [su]mlal intrinsics

aarch64: Use RTL builtins for [su]mlal intrinsics

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

gcc/ChangeLog:

2021-01-26  Jonathan Wright  <jonathan.wright@arm.com>

	* config/aarch64/aarch64-simd-builtins.def: Add [su]mlal
	builtin generator macros.
	* config/aarch64/aarch64-simd.md (*aarch64_<su>mlal<mode>):
	Rename to...
	(aarch64_<su>mlal<mode>): This.
	* config/aarch64/arm_neon.h (vmlal_s8): Use RTL builtin
	instead of inline asm.
	(vmlal_s16): Likewise.
	(vmlal_s32): Likewise.
	(vmlal_u8): Likewise.
	(vmlal_u16): Likewise.
	(vmlal_u32): Likewise.