[LLDB][MIPS] Provide CPU string to compiler for appropriate code generation for MIPS

[LLDB][MIPS] Provide CPU string to compiler for appropriate code generation for MIPS

    SUMMARY:
    This patch implements ArchSpec::GetClangTargetCPU() that provides string representing current architecture as a target CPU.
    This string is then passed to tools like clang so that they generate correct code for that target.

    Reviewers: clayborg, zturner
    Subscribers: mohit.bhakkad, sagar, jaydeep, lldb-commits
    Differential Revision: http://reviews.llvm.org/D17022

llvm-svn: 261206