aarch64: Use RTL builtins for integer mla intrinsics

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

gcc/Changelog:

2021-01-14  Jonathan Wright  <jonathan.wright@arm.com>

	* config/aarch64/aarch64-simd-builtins.def: Add mla builtin
	generator macro.
	* config/aarch64/arm_neon.h (vmla_s8): Use RTL builtin rather
	than asm.
	(vmla_s16): Likewise.
	(vmla_s32): Likewise.
	(vmla_u8): Likewise.
	(vmla_u16): Likewise.
	(vmla_u32): Likewise.
	(vmlaq_s8): Likewise.
	(vmlaq_s16): Likewise.
	(vmlaq_s32): Likewise.
	(vmlaq_u8): Likewise.
	(vmlaq_u16): Likewise.
	(vmlaq_u32): Likewise.