Compiler dosen't need to know about certain settings from file

 - RTS and libdw
 - SMP
 - RTS ways

I am leaving them in the settings file because `--info` currently prints
all the fields in there, but in the future I do believe we should
separate the info GHC actually needs from "extra metadata". The latter
could go in `+RTS --info` and/or a separate file that ships with the RTS
for compile-time inspection instead.