[SystemZ] Add -mbackchain option.

This option, like the corresponding gcc option, is SystemZ-specific and
enables storing frame backchain links, as specified in the ABI.

Differential Revision: http://reviews.llvm.org/D19891

llvm-svn: 268575
diff --git a/clang/lib/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
index 2091ec4..0b154b6 100644
--- a/clang/lib/Frontend/CompilerInvocation.cpp
+++ b/clang/lib/Frontend/CompilerInvocation.cpp
@@ -782,6 +782,8 @@
   Opts.CudaGpuBinaryFileNames =
       Args.getAllArgValues(OPT_fcuda_include_gpubinary);
 
+  Opts.Backchain = Args.hasArg(OPT_mbackchain);
+
   return Success;
 }