Allow overriding -fno-gnu-inline-assembly

While -fno-gnu-inline-assembly is default, in some cases we need want
override the option and explicitly enable inline assembly.

BUG=https://bugs.chromium.org/p/nativeclient/issues/detail?id=4347
R=dschuff@chromium.org

Review URL: https://codereview.chromium.org/1767393002 .

lib/Driver/Tools.cpp

diff --git a/lib/Driver/Tools.cpp b/lib/Driver/Tools.cpp
index dedeb68..f4f7c4e 100644
--- a/lib/Driver/Tools.cpp
+++ b/lib/Driver/Tools.cpp
@@ -1770,7 +1770,10 @@
                               ArgStringList &CmdArgs) const {
   Args.ClaimAllArgs(options::OPT_emit_obj);
 
-  CmdArgs.push_back("-fno-gnu-inline-asm");
+  // -fno-gnu-inline-asm is default.
+  if (!Args.hasFlag(options::OPT_fgnu_inline_asm,
+                    options::OPT_fno_gnu_inline_asm, false))
+    CmdArgs.push_back("-fno-gnu-inline-asm");
 }
 
 // Decode AArch64 features from string like +[no]featureA+[no]featureB+...