[mips] Use pristine object file while processing relocations.

    Similar to ARM change r182800, dynamic linker will read bits/addends from
    the original object rather than from the object that might have been patched
    previously. For the purpose of relocations for MCJIT stubs on MIPS, we
    internally use otherwise unused MIPS relocations.
    
    The change also enables MCJIT unit tests for MIPS (EL/BE), and the following
    two tests now pass:
    
    - MCJITTest.return_global and
    - MCJITTest.multiple_functions.
    
    These issues have been tracked as Bug 16250.

    Patch by Petar Jovanovic.

llvm-svn: 187019