LibC: add wait and waitpid

Note that wait() doesn't work since only waiting for specified
pid is supported. wait() will just return -1 and set errno to
ECHILD.

libc/sys/wait.cpp

@@ -0,0 +1,13 @@
+#include <sys/syscall.h>
+#include <sys/wait.h>
+#include <unistd.h>
+
+pid_t wait(int* stat_loc)
+{
+	return waitpid(-1, stat_loc, 0);
+}
+
+pid_t waitpid(pid_t pid, int* stat_loc, int options)
+{
+	return (pid_t)syscall(SYS_WAIT, pid, stat_loc, options);
+}