[Mips] Teach llvm-readobj to print MIPS-specific ELF program headers.

The patch reviewed by Michael Spencer.
http://llvm-reviews.chandlerc.com/D1846

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@192093 91177308-0d34-0410-b5e6-96231b3b80d8