user: implement mlibc as the libc, finally.

It's finally done..

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

user/include/mlibc/tests/linux/malloc-usable-size.c

@@ -0,0 +1,13 @@
+#include <assert.h>
+#include <malloc.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+int main() {
+	void *p1 = malloc(1023);
+	fprintf(stderr, "size: %zu\n", malloc_usable_size(p1));
+	assert(malloc_usable_size(p1) >= 1023);
+	free(p1);
+
+	return 0;
+}