Extend expiration of ports broken with -fno-common

Extend the expiration of unmaintained leaf ports that are broken with
-fno-common and previously had a short expiration date. The new date is set
to 2021-04-01, meaning the ports will be in the 2021Q2 branch, giving people
until 2021-06-30 (10 months from now) to fix issues.
Improve the wording of the deprecation message, mentioning both llvm 11 and
-fno-common.
Bump portrevision to force reinstall. This way the warning message about
deprecation will be displayed to users installing or reinstalling the
software.

MFH: 2020Q3 (maybe)