CMake: Fix unknown architecture and simplify OSX_ARCHITECTURES

[enetheru]

It was brought to my attention that @Naros was getting builds with the architecture name tag set to unknown

This is cosmetic and doesnt effect the build, but did expose an underlying problem with how I was treating the CMAKE_SYSTEM_PROCESSOR flag, and how I was generating the OUTPUT_NAME

It also made me re-examine how I was treating the OSX_ARCHITECTURES, in light of the recent MSVC_RUNTIME_LIBRARIES #1701 discussion

Changes:

• Remove GODOT_ARCH option, it's managed by the toolchain files, or build vars.
• Rename godot_arch_map to godot_arch_name to better describe its use
• Add the known Android architectures to the godot_arch_name internal lists
• Return ${CMAKE_SYSTEM_PROCESSOR} when architecture is unknown instead of unknown
• Remove all reference to OSX_ARCHITECTURES, and universal and provide links to documentation on how to use OSX_ARCHITEXTURES in the cmake/macos.cmake header

This simplifies the case for MacOS, iOS, and makes the resulting naming more predictable

[dsnopek]

I don't understand a lot of what's here, but I trust @enetheru and @Naros has tested it, so let's give it a shot :-)