Add nacl-clang testing to SCons

src/untrusted/stubs/crti_x86_64.S

Index: src/untrusted/stubs/crti_x86_64.S
diff --git a/src/untrusted/stubs/crti_x86_64.S b/src/untrusted/stubs/crti_x86_64.S
index fa93b737ee9ab08a26f885159536ea320079fc94..704c67534225a601e4c171d41a8355e62e576ed8 100644
--- a/src/untrusted/stubs/crti_x86_64.S
+++ b/src/untrusted/stubs/crti_x86_64.S
@@ -4,6 +4,7 @@
  * found in the LICENSE file.
  */
 
+#if !defined(__clang__)
     /*
      * The .init section contains one symbol, _init, which is the entry
      * of a function and hence needs to be aligned in accordance with
@@ -32,7 +33,23 @@ _init:
 _fini:
     pushq  %rbp
     movq   %rsp, %rbp
+#else
+    /*
+     * nacl-clang does not use the .init/.fini mechanism at all, but newlib
+     * still calls _init and _fini, so define them as empty functions in the
+     * text section.
+     */
+    .section .text
+    .p2align NACLENTRYALIGN
+    .global _init
+_init:
+     naclret
 
+     .p2align NACLENTRYALIGN
+     .global _fini
+_fini:
+     naclret
+#endif
     /*
      * The code that follows will be appended by the linker, and will not
      * be aligned individually, so we need to align it.