Simplify a really complicated check for Arch == X86_64.

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203605 91177308-0d34-0410-b5e6-96231b3b80d8