user: implement mlibc as the libc, finally.

It's finally done..

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

user/include/mlibc/tests/bsd/getloadavg.c

@@ -0,0 +1,10 @@
+#include <assert.h>
+#include <stdlib.h>
+#include <stdio.h>
+
+int main(void) {
+    double samples[3];
+    assert(getloadavg(samples, 3) != 0);
+    printf("%f %f %f\n", samples[0], samples[1], samples[2]);
+    return 0;
+}