Make the default doc index file path compiler/arch/os-dependent.
Make the default doc index file path compiler/arch/os-dependent.

Fixes #2135.

mmakowski/cabal,kosmikus/cabal,typelead/epm,Helkafen/cabal,enolan/cabal,mightybyte/cabal,Heather/cabal,Heather/cabal,ghc-android/cabal,gridaphobe/cabal,martinvlk/cabal,thoughtpolice/cabal,plumlife/cabal,x-y-z/cabal,valderman/cabal,enolan/cabal,eric-stanley/cabal,rimmington/cabal,beni55/cabal,edsko/cabal,edsko/cabal,Peaker/cabal,rimmington/cabal,anton-dessiatov/cabal,randen/cabal,plumlife/cabal,garetxe/cabal,kolmodin/cabal,thoughtpolice/cabal,bennofs/cabal,ian-ross/cabal,mmakowski/cabal,themoritz/cabal,valderman/cabal,Peaker/cabal,themoritz/cabal,plow-technologies/cabal,christiaanb/cabal,Helkafen/cabal,ghc-android/cabal,headprogrammingczar/cabal,mydaum/cabal,x-y-z/cabal,gbaz/cabal,headprogrammingczar/cabal,typelead/epm,triplepointfive/cabal,trskop/cabal,corngood/cabal,gridaphobe/cabal,eric-stanley/cabal,themoritz/cabal,anton-dessiatov/cabal,lukexi/cabal,bennofs/cabal,martinvlk/cabal,sopvop/cabal,randen/cabal,mightybyte/cabal,lukexi/cabal,corngood/cabal,gbaz/cabal,fugyk/cabal,thomie/cabal,seereason/cabal,triplepointfive/cabal,plow-technologies/cabal,sopvop/cabal,ian-ross/cabal,seereason/cabal,trskop/cabal,garetxe/cabal,kosmikus/cabal,thomie/cabal,kolmodin/cabal,fugyk/cabal,beni55/cabal,sopvop/cabal,mydaum/cabal,christiaanb/cabal