[AArch64] Use implementation namespace consistently in arm_neon.h

We're somewhat inconsistent in arm_neon.h when it comes to using the implementation namespace for local
identifiers. This means things like:
#define hash_abcd 0
#define hash_e 1
#define wk 2

#include "arm_neon.h"

uint32x4_t
foo (uint32x4_t a, uint32_t b, uint32x4_t c)
{
  return vsha1cq_u32 (a, b, c);
}

don't compile.
This patch fixes these issues throughout the whole of arm_neon.h
Bootstrapped and tested on aarch64-none-linux-gnu.
The advsimd-intrinsics.exp tests pass just fine.

From-SVN: r276125