[clang][Driver][ARM] Favor -mfpu over default CPU features

When processing the command line options march, mcpu and mfpu, we store
the implied target features on a vector. The change D62998 introduced a
temporary vector, where the processed features get accumulated. When
calling DecodeARMFeaturesFromCPU, which sets the default features for
the specified CPU, we certainly don't want to override the features
that have been explicitly specified on the command line. Therefore, the
default features should appear first in the final vector. This problem
became evident once I added the missing (unhandled) target features in
ARM::getExtensionFeatures.

Differential Revision: https://reviews.llvm.org/D63936

llvm-svn: 366027