POSIX RegisterContext for mips64

Based on the POSIX x86_64 register context.  This is sufficient for opening
a mips64 (big endian) core file.  Subsequent changes will connect the
disassembler, dynamic loader support, ABI, etc.

Review: http://llvm-reviews.chandlerc.com/D1873

(Missed "svn add" on this file in r192335)