user: implement mlibc as the libc, finally.

It's finally done..

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

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

@@ -0,0 +1,31 @@
+#include <stdio.h>
+#include <setjmp.h>
+#include <stdnoreturn.h>
+#include <assert.h>
+
+jmp_buf buf;
+
+noreturn void do_jump(int arg) {
+	longjmp(buf, 2 * arg);
+}
+
+int main(void) {
+	volatile int times_called = 0;
+
+	if (setjmp(buf) != 8) {
+		do_jump(++times_called);
+	}
+
+	assert(times_called == 4);
+
+	times_called = 0;
+	int ret = setjmp(buf);
+	assert(ret == times_called);
+
+	if (!ret) {
+		times_called = 1;
+		longjmp(buf, 0);
+	}
+
+	return 0;
+}