arch.7: add default tool chain table

MFC r351035: arch.7: add tool chain table
MFC r351036: arch.7: add deprecation notice for GCC 4.2.1
MFC r351037: arch.7: clarify that the table shows the default tool chain
MFC r351063: arch.7: be explicit about the future for unmigrated GCC 4.2.1 archs