RtsFlags: Refactor some of the deeper switches
RtsFlags: Refactor some of the deeper switches

This was previously nearly impossible to read; now it's merely
difficult.

Ideally we would do a more thorough refactoring of the RTS command line
parser (#4243) but this is more effort than I have time for at the
moment.

Test Plan: Try using affected RTS flags

Reviewers: simonmar, austin

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1476

GHC Trac Issues: #4243

ezyang/ghc,elieux/ghc,ezyang/ghc,GaloisInc/halvm-ghc,sgillespie/ghc,tjakway/ghcjvm,vTurbine/ghc,tjakway/ghcjvm,snoyberg/ghc,oldmanmike/ghc,olsner/ghc,tjakway/ghcjvm,GaloisInc/halvm-ghc,ezyang/ghc,vikraman/ghc,sdiehl/ghc,vTurbine/ghc,mettekou/ghc,GaloisInc/halvm-ghc,mcschroeder/ghc,nushio3/ghc,snoyberg/ghc,mcschroeder/ghc,snoyberg/ghc,mettekou/ghc,gridaphobe/ghc,oldmanmike/ghc,mettekou/ghc,sgillespie/ghc,sdiehl/ghc,ezyang/ghc,olsner/ghc,gridaphobe/ghc,gridaphobe/ghc,tjakway/ghcjvm,vTurbine/ghc,shlevy/ghc,sdiehl/ghc,elieux/ghc,oldmanmike/ghc,vikraman/ghc,nushio3/ghc,vikraman/ghc,snoyberg/ghc,nushio3/ghc,vikraman/ghc,ezyang/ghc,sdiehl/ghc,gridaphobe/ghc,GaloisInc/halvm-ghc,mettekou/ghc,GaloisInc/halvm-ghc,tjakway/ghcjvm,elieux/ghc,olsner/ghc,sgillespie/ghc,mcschroeder/ghc,sdiehl/ghc,elieux/ghc,nushio3/ghc,oldmanmike/ghc,mettekou/ghc,sdiehl/ghc,GaloisInc/halvm-ghc,sgillespie/ghc,snoyberg/ghc,snoyberg/ghc,nushio3/ghc,mcschroeder/ghc,vikraman/ghc,shlevy/ghc,vTurbine/ghc,elieux/ghc,shlevy/ghc,ezyang/ghc,mcschroeder/ghc,olsner/ghc,mcschroeder/ghc,mettekou/ghc,snoyberg/ghc,oldmanmike/ghc,GaloisInc/halvm-ghc,olsner/ghc,ezyang/ghc,sgillespie/ghc,vTurbine/ghc,sdiehl/ghc,olsner/ghc,mcschroeder/ghc,gridaphobe/ghc,sgillespie/ghc,nushio3/ghc,vikraman/ghc,vikraman/ghc,sgillespie/ghc,gridaphobe/ghc,shlevy/ghc,shlevy/ghc,vTurbine/ghc,gridaphobe/ghc,elieux/ghc,olsner/ghc,nushio3/ghc,oldmanmike/ghc,tjakway/ghcjvm,shlevy/ghc,tjakway/ghcjvm,elieux/ghc,vTurbine/ghc,shlevy/ghc,oldmanmike/ghc,mettekou/ghc