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


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@192335 91177308-0d34-0410-b5e6-96231b3b80d8