This is what we have:
> compel/src/lib/infect.c:1145:38: error: taking address of packed member
> 'uc_sigmask' of class or structure 'ucontext_ia32' may result in an
> unaligned pointer value [-Werror,-Waddress-of-packed-member]
>         blk_sigset = RT_SIGFRAME_UC_SIGMASK(f);
>                      ~~~~~~~~~~~~~~~~~~~~~~~^~
> compel/include/uapi/asm/sigframe.h:133:4: note: expanded from macro
> 'RT_SIGFRAME_UC_SIGMASK'
>         (&rt_sigframe->compat.uc.uc_sigmask))
>           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 1 error generated.

Indeed this results in an unaligned pointer, but as this is intended and
well known (see commit dd6736bd "compel/x86/compat: pack ucontext_ia32"),
we need to silence the warning here.

For more details, see https://reviews.llvm.org/D20561

Originally found by Travis on Alpine Linux, reproduced on Ubuntu 17.10.

[v2: fix for non-x86]
Reported-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>