Basic: use hex string for __ARM_FP macro

The ARM ACLE describes the values as hex constants rather than numeric
constants; follow suit.  Address post-commit review comments from Jon Roelofs.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@218009 91177308-0d34-0410-b5e6-96231b3b80d8