Extend CI to cover latest GCC and Clang

[thesamesam]

Mistakes happen and it's easily done, so no blame intended here! But if CI covered GCC 12, we may well have caught the following early:

• 05e5705
• Build error with gcc 12.1 #155
• Build failure #162
• assuming pointer wraparound does not occur when comparing P +- C1 with P +- C2 [-Werror=strict-overflow] #163

CI is currently running GCC 11.3.0 (which is the latest 11.x release) and Clang "14.0-55" (which is not an upstream release, but fwiw the latest LLVM 14 release was 14.0.6).

I think it's desirable to have GCC 12 and Clang 15 covered in CI. OpenSSL's compiler-zoo.yml (https://github.com/openssl/openssl/blob/master/.github/workflows/compiler-zoo.yml) shows how to get newer GCC and Clang on the Ubuntu images used by github actions.

(For GCC, it's enough to use the latest non-LTS Ubuntu, but for Clang, it uses their own repository. It may be necessary to use another repository for GCC in some cases.)