musl: introduce GNU specific version of strerror_r()

musl provides XSI compliant strerror_r(), and it is slightly different
from the one by glibc.
Let's introduce a tiny wrapper to convert XSI strerror_r() to GNU one.

The wrapper also patches musl's spurious catchall error message.

src/include/musl/string.h

@@ -0,0 +1,7 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+#pragma once
+
+#include_next <string.h>
+
+char* strerror_r_gnu(int errnum, char *buf, size_t buflen);
+#define strerror_r strerror_r_gnu