[X86] Add -mno-iamcu option.

Add -mno-iamcu option to:
  1) Countervail -miamcu option easily
  2) Be compatible with GCC which supports this option

Differential Revision: http://reviews.llvm.org/D21469

llvm-svn: 273147
diff --git a/clang/lib/Driver/Tools.cpp b/clang/lib/Driver/Tools.cpp
index b911c17..a23f660 100644
--- a/clang/lib/Driver/Tools.cpp
+++ b/clang/lib/Driver/Tools.cpp
@@ -2243,10 +2243,12 @@
   }
 
   // Set flags to support MCU ABI.
-  if (Args.hasArg(options::OPT_miamcu)) {
-    CmdArgs.push_back("-mfloat-abi");
-    CmdArgs.push_back("soft");
-    CmdArgs.push_back("-mstack-alignment=4");
+  if (Arg *A = Args.getLastArg(options::OPT_miamcu, options::OPT_mno_iamcu)) {
+    if (A->getOption().matches(options::OPT_miamcu)) {
+      CmdArgs.push_back("-mfloat-abi");
+      CmdArgs.push_back("soft");
+      CmdArgs.push_back("-mstack-alignment=4");
+    }
   }
 }