Silence clang-4 warning about unaligned pointer

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>

compel/arch/x86/src/lib/include/uapi/asm/sigframe.h

@@ -130,7 +130,7 @@ struct rt_sigframe {
 #define RT_SIGFRAME_UC_SIGMASK(rt_sigframe)				\
 	((rt_sigframe->is_native)			?		\
 	(&rt_sigframe->native.uc.uc_sigmask) :				\
-	(&rt_sigframe->compat.uc.uc_sigmask))
+	((k_rtsigset_t *)(void *)&rt_sigframe->compat.uc.uc_sigmask))
 
 #define RT_SIGFRAME_REGIP(rt_sigframe)					\
 	((rt_sigframe->is_native)			?		\

compel/src/lib/infect.c

@@ -1126,7 +1126,7 @@ static int save_regs_plain(void *to, user_regs_struct_t *r, user_fpregs_struct_t
 
 #ifndef RT_SIGFRAME_UC_SIGMASK
 #define RT_SIGFRAME_UC_SIGMASK(sigframe)				\
-	(k_rtsigset_t*)&RT_SIGFRAME_UC(sigframe)->uc_sigmask
+	(k_rtsigset_t*)(void *)&RT_SIGFRAME_UC(sigframe)->uc_sigmask
 #endif
 
 static int make_sigframe_plain(void *from, struct rt_sigframe *f, struct rt_sigframe *rtf, k_rtsigset_t *b)