[MIPS] initFeatureMap() to handle empty string argument

    
    SUMMARY:
    This patch sets CPU string to its default value when it is not supplied by caller.
    
    Reviewers: vkalintiris, dsanders
    Subscribers: mohit.bhakkad, sagar, jaydeep, cfe-commits
    Differential Revision: http://reviews.llvm.org/D16139

llvm-svn: 262691