Add nacl-clang testing to SCons

tests/threads/race_test.c

Index: tests/threads/race_test.c
diff --git a/tests/threads/race_test.c b/tests/threads/race_test.c
index 8fdd1fe6167bd9039ad60599afe995c4ffdbebf1..c4f875866f781942071ea9bc07c5090ec617c856 100644
--- a/tests/threads/race_test.c
+++ b/tests/threads/race_test.c
@@ -52,7 +52,7 @@ NOINLINE void break_optimization(void) {
   volatile int foo;
   foo = 0;

[jvoung (off chromium), 2014/10/08 22:40:23]

Could just make this one case w/out __asm__?

[Derek Schuff, 2014/10/09 16:55:42]

I dunno, an asm seems like a lot more reliable way to defeat the optimizer if
you can have it.
Oh and gcc 4.8 warns on this that foo is set but not used. So we should just
keep it I think.

 #else
-  asm volatile ("");
+  __asm__ volatile ("");
 #endif
 }