compel/x86/compat: pack ucontext_ia32

As I've united k_rtsigset_t between native and compat ucontext's,
it's 8-bytes aligned now. We don't care about align of this as
we fill it always with memcpy()/memset(0).
So after those changes, ucontext_ia32 has received new padding
between uc_mcontext and uc_sigmask (4 bytes to align to 8-byte sized
sigmask). Because of this, mask of blocked signals was restored
with shift (wrongly).
Fixes: signalfd00, file_attr tests.

travis-ci: success for 32-bit tests fixes
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

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

@@ -99,7 +99,7 @@ struct ucontext_ia32 {
 	compat_stack_t		uc_stack;
 	struct rt_sigcontext_32	uc_mcontext;
 	k_rtsigset_t		uc_sigmask; /* mask last for extensibility */
-};
+} __packed;
 
 struct rt_sigframe_ia32 {
 	uint32_t		pretcode;