arm: Add compile-time asserts for siginfo_t offsets

To help catch ABI breaks at compile-time, add compile-time assertions to
verify the siginfo_t layout.

This could have caught that we cannot portably add 64-bit integers to
siginfo_t on 32-bit architectures like Arm before reaching -next:
https://lkml.kernel.org/r/20210422191823.79012-1-elver@google.com

Link: https://lkml.kernel.org/r/20210429190734.624918-2-elver@google.com
Link: https://lkml.kernel.org/r/20210505141101.11519-2-ebiederm@xmission.com
Link: https://lkml.kernel.org/r/87y2a7xx9q.fsf_-_@disp2133


Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>