define macro for stack alignment

Replace stack alignment magic constant with
__stack_aligned__ macro.
Also align stack for sigaltstack test case.

Signed-off-by: Vijaya Kumar K <vijayak@caviumnetworks.com>
Reviewed-by: Christopher Covington <cov@codeaurora.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>