sparc64: 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. Unlike other architectures, sparc64 is
special, because it is one of few architectures requiring si_trapno.
ABI breaks around that field would only be caught here.

Link: https://lkml.kernel.org/r/m11rat9f85.fsf@fess.ebiederm.org
Link: https://lkml.kernel.org/r/20210429190734.624918-1-elver@google.com
Link: https://lkml.kernel.org/r/20210505141101.11519-1-ebiederm@xmission.com
Link: https://lkml.kernel.org/r/874kcvzbuu.fsf_-_@disp2133


Suggested-by: Eric W. Biederman <ebiederm@xmission.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>