[Sparc] Make soft-float emit an error.

LLVM does not and has not ever supported a soft-float ABI mode on
Sparc, so don't pretend that it does.

Also switch the default from "soft-float" -- which was actually
hard-float because soft-float is unimplemented -- to hard-float.

Differential Revision: http://reviews.llvm.org/D10457

llvm-svn: 239755