Enable MSVC cl.exe build on Windows ARM64

[pdeep854]

PR: Enable MSVC (cl.exe) build on Windows ARM64

Summary

This PR restores full build compatibility for xsimd with the MSVC compiler (cl.exe) on
Windows ARM64 (_M_ARM64). Prior to this change, the project failed to compile with MSVC
on ARM64 due to fundamental differences in how MSVC exposes ARM NEON intrinsics compared
to GCC and Clang.

All changes are strictly additive and backward-compatible: every existing GCC/Clang code
path is preserved unchanged inside #else branches. No behaviour is altered for any
non-MSVC-ARM64 target.

Testing

The build was verified locally on a Windows ARM64 machine using Visual Studio 2022:

cmake .. -DBUILD_TESTS=ON -DDOWNLOAD_DOCTEST=ON -DBUILD_BENCHMARK=ON ^
         -DBUILD_EXAMPLES=ON -DCMAKE_BUILD_TYPE=Release ^
         -G "Visual Studio 17 2022"
cmake --build . --config Release

Result: All three targets build successfully with zero errors:

benchmark_xsimd.vcxproj -> ...\Release\benchmark_xsimd.exe
mandelbrot.vcxproj      -> ...\Release\mandelbrot.exe
test_xsimd.vcxproj      -> ...\Release\test_xsimd.exe