[Object] Remove conditional layout of bitfields in MachO::relocation_info.

This removes the conditional layout of relocation_info bitfields that was
introduced in 3ccd677b (svn r358839). The platform relocation_info
struct (defined in usr/include/mach-o/reloc.h) does not define the layout of
this struct differently on big-endian platforms and we want to keep the LLVM
and platform definitions in sync.

To fix the bug that 3ccd677b addressed this patch modifies JITLink to construct
its relocation_info structs from the raw relocation words using shift and mask
operations.