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

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@134691 91177308-0d34-0410-b5e6-96231b3b80d8