Cross-compile for Windows targets using mingw-w64

[tmorlando]

Describe the feature
Implement build system changes and source code changes needed for Windows AMD64 targets.

Supporting Windows platform as a pkcs11-provider target would benefit makers of cross-platform smart-card applications (me included). OpenSSL provides instructions on how to produce libraries using cross-compilation on a Linux-host in file NOTES-WINDOWS.md, section "Native builds using MinGW", and the said changes could follow this approach instead of native Windows builds.

Expected behavior

Existing PKCS#11 smart cards modules can be used from applications using OpenSSL 3.0

Additional context

Non-minimal change-set is available from:
tmorlando@f61f976

Code on src/mutex.c is likely redundant, as MinGW provides pthread compatibility layer for Windows targets.

Adding Windows as a target platform may require substantial changes to CI pipelines cross-compilation results are to be tested.

[simo5]

The code you point at does not look too bad, we'd need to think a bit better about some names and which file helpers belong to but it is overall acceptable should you wish to submit a PR for it.

However a critical problem would be testing. We can use CI to ensure the code builds, but how would we know if it actually works?

[tmorlando]

I read the above as a "no-objection" for a new target platform. However, I'll hold submitting a PR and try to address testability issues with wine or something alike.