Implement clone() C library function properly.

Only provide an implementation for ARM at the moment, since
it requires specific assembly fragments (the standard syscall
stubs cannot be used because the child returns in a different
stack).
diff --git a/libc/SYSCALLS.TXT b/libc/SYSCALLS.TXT
index 684b43e..6a21b04 100644
--- a/libc/SYSCALLS.TXT
+++ b/libc/SYSCALLS.TXT
@@ -34,7 +34,12 @@
 pid_t   __fork:fork (void)           2
 pid_t   _waitpid:waitpid (pid_t, int*, int, struct rusage*)   -1,7
 int     waitid(int, pid_t, struct siginfo_t*, int,void*)          280,284
-pid_t   __clone:clone(int (*fn)(void*), void *child_stack, int flags, void *arg)  120
+
+# NOTE: this system call is never called directly, but we list it there
+#       to have __NR_clone properly defined.
+#
+pid_t   __sys_clone:clone (int, void*, int*, void*, int*) 120
+
 int     execve (const char*, char* const*, char* const*)  11
 
 int     setuid:setuid32 (uid_t)    213