aarch64: Fix restore for multithreaded processes

The stack handling in the RUN_CLONE_RESTORE_FN macro for
AArch64 was incorrect, resulting in a segfault of the restored
process. With this change the pthread00 test case (and probably
others) runs to completion and passes.

Signed-off-by: Christopher Covington <cov@codeaurora.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>