test-gdb: bring back to life

Well factored with ./test-baremetal, allowing for target selection.

userland/gdb_tests/add.c

@@ -0,0 +1,13 @@
+#include <assert.h>
+
+int main(void) {
+    int i, j, k;
+    i = 1;
+    /* test-gdb-op1 */
+    j = 2;
+    /* test-gdb-op2 */
+    k = i + j;
+    /* test-gdb-result */
+    if (k != 3)
+        assert(0);
+}