aarch64: Use mf8 instead of f8 in builtin definitions

The intrinsic type suffix for modal floating-point types is _mf8,
so it's more convenient if we use that for the simd_types as well.

gcc/
	* config/aarch64/aarch64-builtins.cc (simd_types::f8): Rename to...
	(simd_types::mf8): ...this.
	* config/aarch64/aarch64-simd-pragma-builtins.def: Update accordingly.