Fix retainer profiling after SRT overhaul
Fix retainer profiling after SRT overhaul

Recent changes to SRTs (D4632, D4637) also required changes to
RetainerProfile.c. This should hopefully get things working again.

Test Plan: validate with profiling turned on

Reviewers: bgamari, osa1, tdammers, erikd

Subscribers: rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D4707

sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc