New executable format: win64/arm64

This is a placeholder issue for adding win64/arm64 (IMAGE_FILE_MACHINE_ARM64).

As the main UPX authors do not use Windows this is a call for external contributors.

Some UPX source code infrastructure is already in place (see src/p_w64pe_arm64.cpp), and the stubs probably can reuse much of the win64/pe handling logic and actual linux/arm64 decompressor code. Related win64/arm64ec #651 should be taken into consideration.

INFO: host win64/arm64 binaries are available in GitHub Actions CI artifacts via windows arm64-win64-vs2022 and zigcc aarch64-windows-gnu