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.

llvm-svn: 203605