Only define __SIZEOF_FLOAT128__ on x86 as intended in r304012

GCC only defines it on x86.

llvm-svn: 304013
diff --git a/clang/lib/Basic/Targets.cpp b/clang/lib/Basic/Targets.cpp
index 636be1f..6be83d2 100644
--- a/clang/lib/Basic/Targets.cpp
+++ b/clang/lib/Basic/Targets.cpp
@@ -4165,6 +4165,9 @@
   }
   if (CPU >= CK_i586)
     Builder.defineMacro("__GCC_HAVE_SYNC_COMPARE_AND_SWAP_8");
+
+  if (HasFloat128)
+    Builder.defineMacro("__SIZEOF_FLOAT128__", "16");
 }
 
 bool X86TargetInfo::hasFeature(StringRef Feature) const {