[Mips] Emit proper ABI for _mcount calls

When -pg option is present than a call to _mcount is inserted into every
function. However since the proper ABI was not followed then the generated
gmon.out did not give proper results. By inserting needed instructions 
before every _mcount we can fix this.

Differential Revision: https://reviews.llvm.org/D68390

llvm-svn: 374055