user: implement mlibc as the libc, finally.

It's finally done..

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

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

@@ -0,0 +1,16 @@
+#include <stdlib.h>
+#include <assert.h>
+#include <limits.h>
+
+int main(){
+    assert(abs(-10) == 10);
+    assert(abs(2021) == 2021);
+
+    assert(labs(-256) == 256);
+    assert(labs(10034890) == 10034890);
+
+    assert(llabs(-0x2deadbeef) == 0x2deadbeef);
+    assert(llabs(49238706947) == 49238706947);
+
+    return 0;
+}