user: implement mlibc as the libc, finally.

It's finally done..

Signed-off-by: kaguya <vpshinomiya@protonmail.com>

user/include/mlibc/tests/rtld/search-order/libbar.c

@@ -0,0 +1,19 @@
+#include <assert.h>
+#include <dlfcn.h>
+
+#ifdef USE_HOST_LIBC
+#define LIBFOO "libnative-foo.so"
+#else
+#define LIBFOO "libfoo.so"
+#endif
+
+int foo(void);
+
+int bar(void) {
+	return foo();
+}
+
+[[gnu::constructor]] void init(void) {
+	void *libfoo = dlopen(LIBFOO, RTLD_LOCAL | RTLD_NOW);
+	assert(libfoo);
+}