rts: Clarify whitehole logic in threadPaused
rts: Clarify whitehole logic in threadPaused

Previously we would look at the indirectee field of a WHITEHOLE object.
However, WHITEHOLE isn't a sort of indirection and therefore has no
indirectee field.

I encountered this while investigating #13615, although it doesn't fix
that bug.

Test Plan: Validate

Reviewers: simonmar, austin, erikd

Subscribers: rwbarton, thomie

GHC Trac Issues: #13615

Differential Revision: https://phabricator.haskell.org/D3674

shlevy/ghc,sdiehl/ghc,shlevy/ghc,ezyang/ghc,ezyang/ghc,ezyang/ghc,sdiehl/ghc,shlevy/ghc,sdiehl/ghc,ezyang/ghc,shlevy/ghc,shlevy/ghc,sdiehl/ghc,shlevy/ghc,ezyang/ghc,shlevy/ghc,sdiehl/ghc,ezyang/ghc,sdiehl/ghc,ezyang/ghc,sdiehl/ghc