src/nsexec: fix stack pointer alignment exception

src/nsexec: fix stack pointer alignment exception

When test g/317 or g/318 on ARM server, we got a kernel exception:

  kernel: nsexec[8203]: SP Alignment exception: pc=00000000004010a0 sp=00000000005200e8

nsexec gives an unaligned child stack address to clone() system
call sometimes. For making sure it's always aligned, use
"__attribute__((aligned))" extension of GCC (Thanks this suggestion
from Eric sandeen).

Signed-off-by: Zorro Lang <zlang@redhat.com>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>