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/CMakeLists.txt

@@ -10,6 +10,7 @@ set(LIBC_SOURCES
 	stdio.cpp
 	stdlib.cpp
 	string.cpp
+	sys/wait.cpp
 	termios.cpp
 	unistd.cpp
 	math.S