arm: Replace calls to __builtin_vneg* by - in arm_neon.h [PR66791]

gcc/
2020-12-16  Prathamesh Kulkarni  <prathamesh.kulkarni@linaro.org>

	PR target/66791
	* config/arm/arm_neon.h: Replace calls to __builtin_vneg* by - operator
	in vneg intrinsics.
	* config/arm/arm_neon_builtins.def: Remove entry for vneg.