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

include/llvm/MC/MCAssembler.h

@@ -66,8 +66,7 @@ private:
   MCSectionData *Parent;
 
   /// Atom - The atom this fragment is in, as represented by it's defining
-  /// symbol. Atom's are only used by backends which set
-  /// \see MCAsmBackend::hasReliableSymbolDifference().
+  /// symbol.
   MCSymbolData *Atom;
 
   /// @name Assembler Backend Data