libc: add stub alarm

libc/unistd/alarm.c

@@ -0,0 +1,8 @@
+#include <stdio.h>
+
+extern char * _argv_0;
+
+unsigned int alarm(unsigned int seconds) {
+	fprintf(stderr, "%s: alarm requested (%d seconds)\n", _argv_0, seconds);
+	return 0;
+}