Douglas Wilson authored Jan 01, 2021 and Ben Gamari committed Apr 07, 2021

Solves #19147. When n_capabilities > 1 we were not correctly accounting
for gc time for sequential collections. In this case par_n_gcthreads ==
1, however it is not guaranteed that the single gc thread is capability 0.

A similar issue for copied is addressed as well.

(cherry picked from commit f49d6fb2)