Simplify a really complicated check for Arch == X86_64.

The function hasReliableSymbolDifference had exactly one use in the MachO
writer. It is also only true for X86_64. In fact, the comments refers to
"Darwin x86_64" and everything else, so this makes the code match the
comment.

If this is to be abstracted again, it should be a property of
TargetObjectWriter, like useAggressiveSymbolFolding.
[mips] Implement NaCl sandboxing of function calls:

  * Add masking instructions before indirect calls (in MC layer).
  * Align call + branch delay to the bundle end (in MC layer).

Differential Revision: http://llvm-reviews.chandlerc.com/D3032