rts/posix: Ensure that memory commit succeeds
rts/posix: Ensure that memory commit succeeds

Previously we wouldn't check that mmap would succeed. I suspect this may
have been the cause of #14329.

Test Plan: Validate under low-memory condition

Reviewers: simonmar, austin, erikd

Reviewed By: simonmar

Subscribers: rwbarton, thomie

GHC Trac Issues: #14329

Differential Revision: https://phabricator.haskell.org/D4075

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