CMake: Use custom package name

[FtZPetruska]

When refactoring packaging with CPack in #745, I wrongly assumed that the default names would be enough. With the default name, the most glaring issue is that both x64 and arm64 packages for macOS have the same name, meaning when a release is made, whichever finishes last overwrites the first.

This PR manually sets the CPACK_PACKAGE_FILE_NAME to dsda-doom-<version>-<platform>-<arch>.

The arch part has a few caveats that need to be worked around:

• In most cases, CMAKE_SYSTEM_PROCESSOR is enough.
• In Visual Studio, when targeting another architecture (e.g. ARM64), CMAKE_SYSTEM_PROCESSOR holds the architecture of the host. Instead CMAKE_VS_PLATFORM_NAME holds the target architecture.
• On macOS, the target architecture(s) may be set in CMAKE_OSX_ARCHITECTURES, similar to Visual Studio, CMAKE_SYSTEM_PROCESSOR still refers to the host.

The platform name is also manually defined. The default CPACK_PACKAGE_FILE_NAME uses CPACK_SYSTEM_NAME, but this has the following caveats:

• On macOS, it evaluates to Darwin which may not be obvious to some user that this is meant for mac.
• On Windows, it evaluates to win32 when targeting x86 and win64 when targeting any other architecture, including arm.

After this change, here is what release artifacts would look like: https://github.com/FtZPetruska/dsda-doom/releases/tag/custom-package-name-test-release

[Pedro-Beirao]

Look good, thank you!
But now I will have to update my windows script since it expects a different name😄
(ik it looks terrible with the powershell calls, but I dont usually write scripts for windows, so whatever)