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/arch-sh/bionic/clone.S b/libc/arch-sh/bionic/clone.S
index 0bbaecb..9cb19ee 100644
--- a/libc/arch-sh/bionic/clone.S
+++ b/libc/arch-sh/bionic/clone.S
@@ -72,3 +72,8 @@
     .align 2
 0:  .long   __NR_clone
 1:  .long   __thread_entry
+
+/* XXX: TODO: Add __bionic_clone here
+ *            See bionic/bionic_clone.c and arch-arm/bionic/clone.S
+ *            for more details...
+ */
\ No newline at end of file