cmd/compile/internal/test: TestIssue71943 fails with OpenSSL and Darwin backends

[qmuntal]

https://dev.azure.com/dnceng-public/public/_build/results?buildId=1015349&view=ms.vss-test-web.build-test-results-tab&runId=27166476&resultId=152972&paneView=debug

TestIssue71943 test is skipped until we figure out why the Go compiler can't stack-allocate a concatenated string passed to sha256.Sum256 when using the OpenSSL or the Darwin backends.

This test has been added in the upstream Go 1.25 development branch as part of fixing this issue: golang/go#71943.

[qmuntal]

Looks like the compiler can't stack-allocate small concatenated strings if the resulting string is passed to runtime.Pinner. The OpenSSL and Darwin backends recently started using it to avoid allocations in the more general case. More investigation is needed.