Build progress bar does not account for muniversal variant

[ryandesign (Ryan Carsten Schmidt)]

The muniversal variant causes multiple configure/build/destroot phases to occur, one per arch. If a port uses both the muniversal portgroup and the cmake portgroup, for which base now has progress bar support, then the progress bar is inaccurate: it goes from 0 to 100% for the first arch, and then goes from 0 to 100% again for the next arch. This will confuse the user since there is deliberately no other indication to the user that a port uses the muniversal portgroup since it is an implementation detail that the user is not supposed to need to know about.

As I said in comment:ticket:15939:20:

We also need to consider that some builds may have multiple parts. For example, a port that uses cmake and the muniversal portgroup will go from 0% to 100% progress more than once (once for each arch).

Maybe there can be a way for portgroups and advanced ports to control the endpoints of the progress bar. The defaults would be that the progress bar goes from 0 to 100 but the muniversal portgroup could manipulate the endpoints so that for example the first arch of a two-arch build sets the endpoints to 0 and 50 and the second arch sets them to 50 and 100.