user: implement mlibc as the libc, finally.

It's finally done..

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

user/include/mlibc/tests/rtld/scope5/libfoo.c

@@ -0,0 +1 @@
+void foo() {}

user/include/mlibc/tests/rtld/scope5/meson.build

@@ -0,0 +1,5 @@
+libfoo = shared_library('foo', 'libfoo.c')
+test_link_with = [libfoo]
+
+libfoo_native = shared_library('native-foo', 'libfoo.c', native: true)
+test_native_link_with = [libfoo_native]

user/include/mlibc/tests/rtld/scope5/test.c

@@ -0,0 +1,29 @@
+#include <dlfcn.h>
+#include <stdio.h>
+#include <assert.h>
+
+#ifdef USE_HOST_LIBC
+#define LIBFOO "libnative-foo.so"
+#else
+#define LIBFOO "libfoo.so"
+#endif
+
+// We need to have a relocation against foo for DT_NEEDED.
+void foo();
+void bar() { foo(); }
+
+int main() {
+	// In this test, we have exec -> foo (where '->' means 'depends on').
+	// This means that foo is in the global scope due to DT_NEEDED.
+	// We then dlopen it again with RTLD_LOCAL, which should just return
+	// the already-loaded object, but used to crash in the mlibc linker instead.
+
+	void *foo = dlopen(LIBFOO, RTLD_LOCAL | RTLD_NOW);
+	assert(foo);
+	assert(dlsym(foo, "foo"));
+	assert(dlsym(RTLD_DEFAULT, "foo"));
+
+	dlclose(foo);
+
+	return 0;
+}