Add long forms for multi-character short-form flags and possibly deprecate short forms

Many of GHC's long form flags use BSD-style single-dash prefixes (e.g. -keep-llvm-files, -rtsopts). Many others use gnu-style double-dash prefixes (e.g. --version, --supported-languages).

This is arguably more confusing than necessary. IMHO it seems reasonable to add Gnu forms for our BSD-style flags (e.g. add --keep-llvm-files) and consider deprecating the latter at some point in the future.