Non-hermetic Haskell toolchains

[mboes]

Is your feature request related to a problem? Please describe.

Remote build execution does not work well with "configure-like" repository rules, which autodetect paths in the execution environment and create symlinks to those. haskell_register_ghc_bindist() and haskell_register_ghc_nixpkgs() macros both use configure-like repository rules, so are unsuitable for RBE.

Describe the solution you'd like

It ought to be possible to create non-hermetic builds, like go_register_toolchains(version = "host"). For consistency with the existing macros, this could be called,

haskell_register_ghc_host()

This solution is also a catch-all for use cases other than RBE where neither bindists nor Nixpkgs are suitable, say a custom GHC on Windows.