Disable stringop-overflow warnings on s390

Disable stringop-overflow warnings on s390

Recently ci on master has been failing:
https://github.com/openssl/openssl/actions/runs/14234051502/job/39919663876

Its occuring because the s390 gcc compiler is complaining about various
functions attempting to write past the end of an array.

However, I can find no case in which we actually do so in this case.

The problem resolves when we either:
1) Disable the stringop-overflow warning
or
2) disable all loop unrolling optimizations with fno-loop-nest-optimize

Given that asan doesn't report any out of bounds errors on s390 when
built with case (1), and case (2) can be a significant performance hit,
coupled with the fact that gcc on any other platform avoids the same
issue (s390 is stuck on gcc 12, instead of gcc 16 where the other
platforms are), I think the right thing to do is just disable the
warning here

Reviewed-by: Kurt Roeckx <kurt@roeckx.be>
Reviewed-by: Tim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/27253)