cgo: support errno value as second return parameter

Making this work on all targets was interesting but there's now a test
in place to make sure this works on all targets that have the CGo test
enabled (which is almost all targets).

testdata/cgo/main.c

@@ -77,3 +77,8 @@ double doSqrt(double x) {
 void printf_single_int(char *format, int arg) {
 	printf(format, arg);
 }
+
+int set_errno(int err) {
+	errno = err;
+	return -1;
+}