Call powerpc-darwin external tools with -arch ppc.

With this patch we call external tools for powerpc-darwin with "-arch ppc"
instead of "-arch powerpc", so as to be compatible with the cctools assembler
and ld64 linker.

Patch by Stephen Drake!

llvm-svn: 216687
diff --git a/clang/lib/Driver/ToolChains.cpp b/clang/lib/Driver/ToolChains.cpp
index 2f47115..f18b6fe 100644
--- a/clang/lib/Driver/ToolChains.cpp
+++ b/clang/lib/Driver/ToolChains.cpp
@@ -156,7 +156,7 @@
 StringRef MachO::getMachOArchName(const ArgList &Args) const {
   switch (getTriple().getArch()) {
   default:
-    return getArchName();
+    return getDefaultUniversalArchName();
 
   case llvm::Triple::aarch64:
     return "arm64";