aarch64: Use memcpy to copy vector tables in vtbx4 intrinsics

aarch64: Use memcpy to copy vector tables in vtbx4 intrinsics

Use __builtin_memcpy to copy vector structures instead of building
a new opaque structure one vector at a time in each of the vtbx4
Neon intrinsics in arm_neon.h. This simplifies the header file and
also improves code generation - superfluous move instructions were
emitted for every register extraction/set in this additional
structure.

gcc/ChangeLog:

2021-07-19  Jonathan Wright  <jonathan.wright@arm.com>

	* config/aarch64/arm_neon.h (vtbx4_s8): Use __builtin_memcpy
	instead of constructing __builtin_aarch64_simd_oi one vector
	at a time.
	(vtbx4_u8): Likewise.
	(vtbx4_p8): Likewise.