add a separate tcc zerosign test which was sent to tcc mailing list

misc/tcc_zerosign2.c

@@ -0,0 +1,9 @@
+#include <stdio.h>
+#include <math.h>
+int main(int argc, char *argv[]) {
+  double d = 0.0;
+  printf("signbit: %d\n", signbit(d));
+  d = -d;
+  printf("signbit: %d\n", signbit(d));
+}
+