SCons: Fix handling of platform-specific tools, notably `mingw`

Add optional `detect.py` `get_tools` method to let platforms register SCons
tools they need.

This helps move this logic out of SConstruct, keeping platforms more self
contained, and helping thirdparty platforms define their own custom tools.

This logic was also unreliable (the `use_mingw` one would only work if
passed manually on the command line, not in e.g. `get_flags`).

Co-authored-by: Thaddeus Crews <repiteo@outlook.com>