testsuite/conc059: Don't attempt to use stdcall where it isn't supported
testsuite/conc059: Don't attempt to use stdcall where it isn't supported

Test Plan: Validate on 64-bit Windows

Reviewers: geekosaur, austin, erikd, Phyx

Reviewed By: Phyx

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2781

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