[mips] Rename data members and member functions in MipsAssemblerOptions.

Summary: Use the naming convention from the LLVM Coding Standards.

Reviewers: dsanders

Reviewed By: dsanders

Differential Revision: http://reviews.llvm.org/D4972

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217254 91177308-0d34-0410-b5e6-96231b3b80d8