[Fuchsia] Use thread-pointer ABI slots for stack-protector and safe-stack

The Fuchsia ABI defines slots from the thread pointer where the
stack-guard value for stack-protector, and the unsafe stack pointer
for safe-stack, are stored. This parallels the Android ABI support.

Patch by Roland McGrath

Differential Revision: https://reviews.llvm.org/D30237

llvm-svn: 296081
diff --git a/llvm/lib/Target/X86/X86ISelDAGToDAG.cpp b/llvm/lib/Target/X86/X86ISelDAGToDAG.cpp
index 7cb7ce1..c3a5346 100644
--- a/llvm/lib/Target/X86/X86ISelDAGToDAG.cpp
+++ b/llvm/lib/Target/X86/X86ISelDAGToDAG.cpp
@@ -719,7 +719,8 @@
   // For more information see http://people.redhat.com/drepper/tls.pdf
   if (ConstantSDNode *C = dyn_cast<ConstantSDNode>(Address))
     if (C->getSExtValue() == 0 && AM.Segment.getNode() == nullptr &&
-        (Subtarget->isTargetGlibc() || Subtarget->isTargetAndroid()))
+        (Subtarget->isTargetGlibc() || Subtarget->isTargetAndroid() ||
+         Subtarget->isTargetFuchsia()))
       switch (N->getPointerInfo().getAddrSpace()) {
       case 256:
         AM.Segment = CurDAG->getRegister(X86::GS, MVT::i16);