[X86][PowerPC] Support -mlong-double-128

This patch makes the driver option -mlong-double-128 available for X86
and PowerPC. The CC1 option -mlong-double-128 is available on all targets
for users to test on unsupported targets.

On PowerPC, -mlong-double-128 uses the IBM extended double format
because we don't support -mabi=ieeelongdouble yet (D64283).

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D64277

llvm-svn: 365866
diff --git a/clang/lib/Basic/Targets/PPC.cpp b/clang/lib/Basic/Targets/PPC.cpp
index b052ef4..bd0ae60 100644
--- a/clang/lib/Basic/Targets/PPC.cpp
+++ b/clang/lib/Basic/Targets/PPC.cpp
@@ -465,6 +465,8 @@
   if (HasAltivec)
     Opts.AltiVec = 1;
   TargetInfo::adjust(Opts);
+  if (LongDoubleFormat != &llvm::APFloat::IEEEdouble())
+    LongDoubleFormat = &llvm::APFloat::PPCDoubleDouble();
 }
 
 ArrayRef<Builtin::Info> PPCTargetInfo::getTargetBuiltins() const {