Fix PCRE2 implementation and tests

[straight-shoota]

Resolves #13103

[beta-ziliani]

Odd, aarch60-musl-test-stdlib is failing without any useful information 🤔

[straight-shoota]

Yeah, that's odd. Will need to investigate further. But I think we should merge this regardless (the test is broken without this PR also) to fix all the other broken tests.
aarch64-musl-test-stdlib succeeded in #13088 (https://github.com/crystal-lang/crystal/actions/runs/4213098782/jobs/7312598177) and on https://github.com/crystal-lang/crystal/tree/ci/test-pcre2-fixes (https://github.com/crystal-lang/crystal/actions/runs/4234860564/jobs/7357786827).

[beta-ziliani]

Thanks a bunch Johannes! 🙇

[straight-shoota]

I checked std_spec on aarch64_linux-musl manually and it runs fine. So that's good.
No idea why it's failing in CI.

[straight-shoota]

Since this was merged, aarch64_linux-musl succeeded in the latest CI builds on master. I guess it was a fluke then (although it appeared repeatedly, so let's keep our eyes open).