Add `_unlock_file` to RTS symbols
Add `_unlock_file` to RTS symbols

Reviewers: bgamari, austin, erikd, simonmar

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2818

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