[mips] Add runtime options to enable/disable madd.fmt and msub.fmt

Add options to clang: -mmadd4 and -mno-madd4, use it to enable or disable
generation of madd.fmt and similar instructions respectively, as per GCC.

Patch by Stefan Maksimovic.

Differential Revision: https://reviews.llvm.org/D33401

llvm-svn: 304929