[Sparc] Add software float option -msoft-float

Summary:
Following patch D19265 which enable software floating point support in the Sparc backend, this patch enables the option to be enabled in the front-end using the -msoft-float option.

The user should ensure a library (such as the builtins from Compiler-RT) that includes the software floating point routines is provided.

Reviewers: jyknight, lero_chris

Subscribers: jyknight, cfe-commits

Differential Revision: http://reviews.llvm.org/D20419