WIP: Implement seccomp-bpf sandbox for nacl_helper_nonsfi.

sandbox/linux/seccomp-bpf/syscall.h

Index: sandbox/linux/seccomp-bpf/syscall.h
diff --git a/sandbox/linux/seccomp-bpf/syscall.h b/sandbox/linux/seccomp-bpf/syscall.h
index dfb44ebdcd780a1ac193187de0ea36cedfd2b02a..ef9bc4bf4e3e71b6f85f23880eeb950aecf3f117 100644
--- a/sandbox/linux/seccomp-bpf/syscall.h
+++ b/sandbox/linux/seccomp-bpf/syscall.h
@@ -8,11 +8,15 @@
 #include <signal.h>
 #include <stdint.h>
 
+// Unlike other header files, this must be include at first, otherwise
+// OS_ prefixed macros are not properly defined.
+#include "build/build_config.h"
+
 #include "base/macros.h"
 #include "sandbox/sandbox_export.h"
 
 // Android's signal.h doesn't define ucontext etc.
-#if defined(OS_ANDROID)
+#if defined(OS_ANDROID) || defined(OS_NACL_NONSFI)
 #include "sandbox/linux/system_headers/android_ucontext.h"
 #endif