runtime: replace hw.ncpu with hw.logicalcpu for Mac OS X
runtime: replace hw.ncpu with hw.logicalcpu for Mac OS X

Reviewed By: erikd, austin

Differential Revision: https://phabricator.haskell.org/D2082

GHC Trac Issues: #8594

vikraman/ghc,olsner/ghc,ezyang/ghc,mettekou/ghc,sgillespie/ghc,mettekou/ghc,vTurbine/ghc,vikraman/ghc,tjakway/ghcjvm,olsner/ghc,vikraman/ghc,vikraman/ghc,snoyberg/ghc,olsner/ghc,tjakway/ghcjvm,mettekou/ghc,vikraman/ghc,shlevy/ghc,ezyang/ghc,tjakway/ghcjvm,ezyang/ghc,olsner/ghc,shlevy/ghc,vTurbine/ghc,ezyang/ghc,shlevy/ghc,sdiehl/ghc,sgillespie/ghc,snoyberg/ghc,tjakway/ghcjvm,mettekou/ghc,vTurbine/ghc,snoyberg/ghc,mettekou/ghc,sdiehl/ghc,vTurbine/ghc,shlevy/ghc,shlevy/ghc,shlevy/ghc,sgillespie/ghc,vTurbine/ghc,sdiehl/ghc,vikraman/ghc,sgillespie/ghc,sgillespie/ghc,snoyberg/ghc,sgillespie/ghc,olsner/ghc,snoyberg/ghc,olsner/ghc,vTurbine/ghc,tjakway/ghcjvm,tjakway/ghcjvm,sdiehl/ghc,shlevy/ghc,sdiehl/ghc,ezyang/ghc,ezyang/ghc,sdiehl/ghc,vikraman/ghc,sgillespie/ghc,tjakway/ghcjvm,vTurbine/ghc,mettekou/ghc,mettekou/ghc,sdiehl/ghc,ezyang/ghc,snoyberg/ghc,snoyberg/ghc,olsner/ghc