aarch64: Delete __builtin_aarch64_neg* builtins and their use

I don't think we need to keep the __builtin_aarch64_neg* builtins around.
They are only used once in the vnegh_f16 intrinsic in arm_fp16.h and I AFAICT
it was added this way only for the sake of orthogonality in
https://gcc.gnu.org/g:d7f33f07d88984cbe769047e3d07fc21067fbba9
We already use normal "-" negation in the other vneg* intrinsics, so do so here as well.

Bootstrapped and tested on aarch64-none-linux-gnu.

gcc/ChangeLog:

	* config/aarch64/aarch64-simd-builtins.def (neg): Delete builtins
	definition.
	* config/aarch64/arm_fp16.h (vnegh_f16): Reimplement using normal negation.