GPS: simplify Default Builder preference
The Default Builder preference has been simplified, with the following changes to the settings:

  - 'Gprmake' has been removed, since gprmake is no longer supported
  - 'Gprbuild' has been renamed 'Auto', which is a more accurate description
  - 'Gprbuild_Always' has been renamed 'Gprbuild', for consistency with the
    'Gnatmake' setting.

Note that if you were using a non-default setting for this preference, you will need to reset it manually.