Avoid resetting the UseSoftFloat and FloatABIType on the TargetMachine

Options struct and move the comment to inMips16HardFloat. Use the
fact that we now know whether or not we cared about soft float to
set the libcalls.
Accordingly rename mipsSEUsesSoftFloat to abiUsesSoftFloat and
propagate since it's no longer CPU specific.

llvm-svn: 213335