[LLDB][MIPS] Handle PIC calling convention for MIPS32

    
    SUMMARY:
    - PrepareTrivialCall() to setup register r25 with the address of function to be called.
    - RegisterIsCalleeSaved() to use name of a register instead of its byte_offset.
    
    Reviewers: clayborg
    Subscribers: mohit.bhakkad, sagar, jaydeep, lldb-commits
    Differential Revision: http://reviews.llvm.org/D15273

llvm-svn: 255005