release: sprinkle UNAME_r hacks following c883b6fd8ccd

Pass UNAME_r override to make(1) for ports builds when building
ports for another branch.

MFC after: 3 days
MFC with: 0be274d37379, 80ab50e1de19, c883b6fd8ccd
Sponsored by: Rubicon Communications, LLC ("Netgate")