rts: Ignore RLIMIT_AS if it is zero
rts: Ignore RLIMIT_AS if it is zero

Reviewers: erikd, simonmar

Reviewed By: simonmar

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #14492

Differential Revision: https://phabricator.haskell.org/D4811

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