[PATCH][MC/X86_64] Implement Win64 exception handling

[Nico Rieck]

Hello Kai!

+  case X86::LEA64r:
...
+    if (SReg == RI->getStackRegister()) {
+      assert(Offset < 0 && "Offset must be negativ");

While fixing PR16779 I noticed that I had this backwards. Offsets here
actually must be *positive*! Sorry for the confusion. So LEA cannot be
used instead of a SUB, and I'll disallow it for atom on Win64.

-Nico