ghc.bootstrapAvailable: init

This will give packages that e.g. want to optionally disable pandoc
based on availability a simple attribute to check, rather than having
to copy quite a complex conditional between packages.

pkgs/development/compilers/ghc/8.10.7.nix

···
 
       # Our Cabal compiler name
       haskellCompilerName = "ghc-${version}";
+
+      bootstrapAvailable = lib.meta.availableOn stdenv.buildPlatform bootPkgs.ghc;
     };
 
     meta = {

pkgs/development/compilers/ghc/common-hadrian.nix

···
       # TODO(@sternenseemann): there's no stage0:exe:haddock target by default,
       # so haddock isn't available for GHC cross-compilers. Can we fix that?
       hasHaddock = stdenv.hostPlatform == stdenv.targetPlatform;
+
+      bootstrapAvailable = lib.meta.availableOn stdenv.buildPlatform bootPkgs.ghc;
     };
 
     meta = {

pkgs/development/compilers/ghc/common-make-native-bignum.nix

···
 
       # Our Cabal compiler name
       haskellCompilerName = "ghc-${version}";
+
+      bootstrapAvailable = lib.meta.availableOn stdenv.buildPlatform bootPkgs.ghc;
     };
 
     meta = {