Move get_cpu guessing into _possible_lib_location

[misl6]

• Not all the platforms need a cpu while guessing for libjvm
• Adding a mapping in MACHINE2CPU for platforms where the cpu doesn't need to be guessed may lead to unexpected bugs in future.
• I noticed it while working on PR Uses cibuildwheel for releases #625 , as tests on Apple Silicon are failing due to:

   self = <json.decoder.JSONDecoder object at 0x101d05ea0>
    [235](https://github.com/kivy/pyjnius/runs/6739396035?check_suite_focus=true#step:12:236)
    s = 'WARNING: Not able to assign machine() = arm64 to a cpu value!\n         Using cpu = \'arm64\' instead!\n["-Dtest.var0...=value", "-Dtest.var35=value", "-Dtest.var36=value", "-Dtest.var37=value", "-Dtest.var38=value", "-Dtest.var39=value"]'
    [236](https://github.com/kivy/pyjnius/runs/6739396035?check_suite_focus=true#step:12:237)
    idx = 0
  

  (basically the test parses what can be found onstdout as a json, but it fails as an unexpected WARNING log is also on stdout buffer)
  The test may be changed in order to avoid this kind of edge case, but this time worked great to improve our codebase 😄 .