ARM: define & use __ARM_NEON on ARM32 (as per ACLE)

There seem to be quite a few references to the old macro __ARM_NEON__ on the
internet, so I don't think it's a good idea to remove it entirely (at least
yet), but the canonical name does not have the trailing underscores so we
should use that ourselves.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@195353 91177308-0d34-0410-b5e6-96231b3b80d8