Fix a FIXME in clang ARM driver that was exposed as a bug with ARM backend

change.

Previously clang was passing the following feature strings to the ARM backend
when CPU is cortex-a8: +neon,-vfp2,-vfp3

This used to work because -vfp2,-vfp3 had no effect after +neon. Now that the
features are controlled by individual bits (with implied hierarchy), the net
effect is all three features will be turned off.

llvm-svn: 134691