runtime: also disable split stacks for runtime_snprintf function under Clang

From-SVN: r212862

libgo/runtime/print.c

@@ -76,9 +76,15 @@ runtime_prints(const char *s)
 // x86-64. Note that signal handlers receive slightly less stack space than they
 // would normally do if they happen to be called while this function is being
 // run. If this turns out to be a problem we could consider increasing BACKOFF.
+
 void
 runtime_printf(const char *s, ...)
 __attribute__((no_split_stack));
+
+int32
+runtime_snprintf(byte *buf, int32 n, const char *s, ...)
+__attribute__((no_split_stack));
+
 #endif
 
 void