ARM: fix VFP asm constraints

Summary:
Rich Felker was sad that clang used 'w' and 'P' for VFP constraints when GCC documents them as 't' and 'w':
  https://gcc.gnu.org/onlinedocs/gcc/Machine-Constraints.html

This was added way back in 2008:
  http://lists.llvm.org/pipermail/cfe-commits/Week-of-Mon-20080421/005393.html

Subscribers: aemerson, rengolin, cfe-commits

Differential Revision: http://reviews.llvm.org/D17349

llvm-svn: 261309