Enable Windows 7 compat path on MinGW builds

[justsmth]

Issues:

Addresses aws/aws-lc-rs#1111

Description of changes:

crypto/rand_extra/windows.c gates the Windows 7 compatibility branch (AWSLC_WINDOWS_7_COMPAT, which uses BCryptGenRandom instead of ProcessPrng) on:

#if !defined(__MINGW32__) && defined(_WIN32_WINNT) && _WIN32_WINNT <= _WIN32_WINNT_WIN7

The !defined(__MINGW32__) guard was suppressing that branch on MinGW even though the top-level CMakeLists.txt already forces -D_WIN32_WINNT=_WIN32_WINNT_WIN7 for MINGW AND NOT CLANG. So MinGW builds were effectively configured to target Windows 7 but then took the ProcessPrng/bcryptprimitives.dll path, which aborts at runtime on Windows 7 because GetProcAddress for ProcessPrng returns NULL there.

This change drops the MinGW guard so the Win7 path is taken when _WIN32_WINNT targets Win7, regardless of toolchain, and adds an explicit target_link_libraries(... bcrypt) for MinGW in crypto/CMakeLists.txt. The link is required because MinGW ignores the #pragma comment(lib, "bcrypt.lib") that MSVC uses inside the same file.

Call-outs:

• The bcrypt link is added unconditionally on MinGW rather than mirroring the preprocessor condition, since detecting the effective _WIN32_WINNT at configure time is fragile. bcrypt.dll exists on all supported Windows versions, and the extra PE import is negligible when the Win7 branch isn't active.
• The existing cross-mingw CI job (which sets _WIN32_WINNT=_WIN32_WINNT_WIN7 via the top-level CMakeLists.txt) will now naturally exercise AWSLC_WINDOWS_7_COMPAT; no new CI job is needed.
• A follow-up is needed in aws-lc-rs to handle the x86_64-win7-windows-gnu Rust target in its builder once this change is pulled in via submodule bump.

Testing:

Local cross-compile from macOS with x86_64-w64-mingw32-gcc 14.0.0:

• Static libcrypto.a builds; nm on rand_extra/windows.c.obj shows only U BCryptGenRandom.
• Shared libcrypto.dll links cleanly; objdump -x shows bcrypt.dll / BCryptGenRandom in the import table and no bcryptprimitives.dll / ProcessPrng references.

The existing cross-mingw CI job will now exercise this code path end-to-end under Wine (Wine implements BCryptGenRandom).

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.14%. Comparing base (9651480) to head (c3aaed1).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3239      +/-   ##
==========================================
+ Coverage   78.12%   78.14%   +0.02%     
==========================================
  Files         689      689              
  Lines      123214   123214              
  Branches    17137    17136       -1     
==========================================
+ Hits        96257    96286      +29     
+ Misses      26047    26018      -29     
  Partials      910      910              

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