man/systemd-gpt-auto-generator: export type flags by name, trim table

man/systemd-gpt-auto-generator: export type flags by name, trim table

We listed many of the root-partition type uuids, but not all. I think the
chances of somebody using the discoverable partition magic on an Itanium system
are fairly low at this point. So let's remove all architectures except the two
most popular consumer architectures. People building Longsoon or RISC-V images
are likely to have the online version of the Discoverable Partitions
Specifications handy anyway, and it makes the man page easier to read. It also
reduces the chances that somebody adding new entries to the table has some
select&paste mishap. We've already had this happen in the past and it's fairly
nasty bug. We don't need to expose the information in two places with manual
propagation between them.