[LLDB][MIPS] Change ARCHFLAG for MIPS

    SUMMARY:
    For MIPS, ARCH is specified without m.

    Reviewers: clayborg
    Subscribers: nitesh.jain, mohit.bhakkad, sagar, bhushan and lldb-commits
    Differential Revision: http://reviews.llvm.org/D14978

llvm-svn: 254376