user: implement mlibc as the libc, finally.

It's finally done..

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

user/include/mlibc/tests/ansi/strxfrm.c

@@ -0,0 +1,16 @@
+#include <assert.h>
+#include <locale.h>
+#include <string.h>
+
+int main() {
+	setlocale(LC_ALL, "C");
+
+	const char *buf = "cbtteststring";
+	char dest[14];
+	size_t ret = strxfrm(dest, buf, strlen(buf) + 1);
+	assert(ret == 13);
+	int cmp = strncmp(dest, buf, 13);
+	assert(!cmp);
+
+	return 0;
+}