rts/RetainerProfile: Handle BLOCKING_QUEUES

push() considers BLOCKING_QUEUES to be an invalid closure type which
should never be present on the stack. However, retainClosure made no
accomodation for this and ended up pushing such a closure. This lead
to #14947.

Test Plan: Validate

Reviewers: simonmar, erikd

Reviewed By: simonmar

Subscribers: thomie, carter, RyanGlScott

GHC Trac Issues: #14947

Differential Revision: https://phabricator.haskell.org/D4538

(cherry picked from commit d5f6d7a0)