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
llvm-svn: 192335

lldb/source/Target/Thread.cpp

@@ -2017,6 +2017,7 @@ Thread::GetUnwinder ()
             case llvm::Triple::x86:
             case llvm::Triple::arm:
             case llvm::Triple::thumb:
+            case llvm::Triple::mips64:
                 m_unwinder_ap.reset (new UnwindLLDB (*this));
                 break;