arm asm: Let .fpu enable instructions, PR20447.

I'm not very happy with duplicating the fpu->feature mapping in ARMAsmParser.cpp
and in clang's driver. See the bug for a patch that doesn't do that, and the
review thread [1] for why this duplication exists.

1: http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20140811/231052.html