[Targets] Move soft-float-abi filtering to `initFeatureMap`

ARM has a special target feature called soft-float-abi. This feature is
special, since we get it passed to us explicitly in the frontend, but
filter it out before it can land in any target feature strings in LLVM
IR.

__attribute__((target(""))) doesn't quite filter these features out
properly, so today, we get warnings about soft-float-abi being an
unknown feature from the backend.

This CL has us filter soft-float-abi out at a slightly different point,
so we don't end up passing these invalid features to the backend.

Differential Revision: https://reviews.llvm.org/D61750

llvm-svn: 363346