MC/Mach-O/x86_64: Add relocation support.

 - This is "extraordinarily" Darwin 'as' compatible. See the litany of FIXMEs littered about for more information.
 - There are a few cases which seem to clearly be 'as' bugs which I have left unsupported, and there is one cases where we diverge but should fix if it blocks diffing .o files (Darwin 'as' ends up widening a jump unnecessarily).
 - 403.gcc build, runs, and diffs equivalently to the 'as' built version now (using llvm-mc). However, it builds so slowly that I wouldn't recommend trying it quite yet. :)

llvm-svn: 98974