[openssl] support non-MSVC compilers on Windows via the unix flow

[omartijn]

The port's Windows path always builds through nmake with cl, bypassing toolchains where the triplet selects another compiler (e.g. clang through a chainloaded toolchain, or cross builds from hosts without MSVC tools — the nmake flow cannot run there at all). Same situation as libsodium's msbuild path in #52803.

This PR detects the configured compiler with vcpkg_cmake_get_vars and routes non-cl.exe compilers through the existing unix flow, adding a clang-windows-msvc Configure target that supplies what the environment cannot: the target triple, Windows system defines, CRT selection, Windows link libraries, and the mingw64 perlasm scheme — clang assembles the gas-syntax perlasm output directly to COFF, so the assembly crypto (AES-NI, AVX-512, SHA extensions) stays enabled. Modules/engine DSOs are disabled for this target; the guard limits it to x64 (other architectures would need their own Configure targets).

MSVC and non-Windows builds are unchanged — the nmake flow and its install-pc-files post-processing are byte-identical, only reached through the new compiler check.

Tested: openssl 3.6.3 cross-compiled for x64-windows from a Linux host with clang 22 through this exact port: configure/build/install green for release and debug, producing COFF libcrypto.a/libssl.a with the perlasm objects included (verified T aesni_cbc_encrypt and the aesni/vpaes/avx512 object files in the archive). Related: #52803, #52808.

• Changes comply with the maintainer guide
• SHA512s are updated for each updated download — n/a, no source change
• ./vcpkg x-add-version openssl was run (3.6.3#1)

[omartijn]

The x64_linux failures (mathgl, octave) are unrelated to this change: both link system Xft through FLTK, and /usr/lib/x86_64-linux-gnu/libXft.so is missing on the current x64-linux CI image — they were only rebuilt here because the openssl port-version bump cascades through their dependency cone. This PR changes nothing on Linux, and all Windows/OSX/Android/arm jobs pass. Filed as #52825 with the evidence.