[PowerPC] Enable soft-float for PPC64, and +soft-float -> -hard-float

Enable soft-float support on PPC64, as the backend now supports it. Also, the
backend now uses -hard-float instead of +soft-float, so set the target features
accordingly.

Fixes PR26970.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@283061 91177308-0d34-0410-b5e6-96231b3b80d8