cmake: Detect and log MinGW-w64 version when cross-compiling for Windows #36029

The behaviour of cross-compiled Windows binaries depends heavily on the toolchain components, including MinGW-w64. When the toolchain is not provided by a distro package manager, determining the exact version being used can be difficult.

This PR detects and logs the MinGW-w64 version when cross-compiling for Windows, simplifying debugging when issues like this arise.

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/36029.

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tACK 09cdb3c48872b7a89a70abfaba86830655cc5007

Cross-compiled for Windows on Arch Linux and the configure summary reports:

Cross compiling ....................... TRUE, for Windows, x86_64, using MinGW-w64 14.0.0

which matches the mingw-w64 version installed on my system.