x32, siginfo: Provide proper overrides for x32 siginfo_t

Provide the proper override macros for x32 siginfo_t.  The combination
of a special type here and an overall alignment constraint actually
ends up with all the types being properly aligned, but the hack is
needed to keep the substructures inside siginfo_t from adding padding.

Note: use __attribute__((aligned())) since __aligned() is not exported
to user space.

[ v2: fix stray semicolon ]

Reported-by: H.J. Lu <hjl.rools@gmail.com>
Cc: Bruce J. Beare <bruce.j.beare@intel.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Link: http://lkml.kernel.org/r/CAMe9rOqF6Kh6-NK7oP0Fpzkd4SBAWU%2BG53hwBbSD4iA2UzyxuA@mail.gmail.com
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>