ARM: allow both vfma and vfms intrinsics on v7.

The main purpose here is that vfma/vfms should be symmetric, and they are
supported on most v7 cores.

The new ArchGuard is suggested by ACLE but prophylactic for us. Almost all CPUs
with NEON *will* have vfma, and the few exceptions I know of (e.g. Cortex-A8)
are incorrectly modelled by Clang so can't trigger a test.

Fortunately, they're getting rarer. But if we ever do support them properly
arm_neon.h should now do the right thing.

llvm-svn: 259537