Check that there are no library modules that use the old "sp" symbol instead

of "c_sp".

.github/checks/checksp.sh

@@ -0,0 +1,22 @@
+#! /bin/bash
+OD65_EXE=../bin/od65
+CHECK_PATH=../../libwrk
+
+cd "${CHECK_PATH}" || {
+    echo "error: Directory ${CHECK_PATH} doesn't seem to exist" >&2
+    exit 1
+}
+
+[ -x "${OD65_EXE}" ] || {
+    echo "error: This check requires the od65 executable to be built" >&2
+    exit 1
+}
+
+EXITCODE=0
+find . -name \*.o -print | while read OBJ; do
+    "${OD65_EXE}" --dump-imports "${OBJ}" | grep -q "\"sp\"" && {
+        echo "error: Usage of symbol 'sp' found in module ${OBJ}" >&2
+        EXITCODE=1
+    }
+done
+exit ${EXITCODE}