user: implement mlibc as the libc, finally.

It's finally done..

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

user/include/mlibc/tests/posix/abort.c

@@ -0,0 +1,21 @@
+#include <signal.h>
+#include <assert.h>
+#include <stdlib.h>
+
+void handler(int sig, siginfo_t *info, void *ctx) {
+	(void)sig;
+	(void)info;
+	(void)ctx;
+}
+
+int main() {
+	struct sigaction sa;
+	sa.sa_sigaction = handler;
+	sa.sa_flags = SA_SIGINFO;
+	sigemptyset(&sa.sa_mask);
+
+	int ret = sigaction(SIGABRT, &sa, NULL);
+	assert(!ret);
+
+	abort();
+}