Remove GHC_STAGE guards from MachDeps
This allows the stage1 compiler (which needs to run on the build
platform and produce code for the host) to depend upon properties of the
target. This is wrong. However, it's no more wrong than it was
previously and @Erichson2314 is working on fixing this so I'm going to
remove the guard so we can finally bootstrap HEAD with ghc-8.8 (see
issue Trac #17146).