[MCJIT] Fix a bug RuntimeDyldImpl's read/writeBytesUnaligned methods.

The previous implementation was writing to the high-bytes of integers on BE
targets (when run on LE hosts).

http://llvm.org/PR20640



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217325 91177308-0d34-0410-b5e6-96231b3b80d8