Don't build wxOSX/Cocoa twice in wxOSX buildbot.

Both trunk builds were using Cocoa, change the one supposed to be Carbon to
use Carbon instead.

Also add Carbon stable branch build as this is probably even more important
than testing Carbon in the trunk.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75358 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775