ghc: compile with DWARF support by default

This allows our GHCs to build programs with DWARF debug information
when -g is passed, see https://ghc.haskell.org/trac/ghc/wiki/DWARF.

Compiling with debug symbols is off by default until GHC ticket
https://gitlab.haskell.org/ghc/ghc/issues/15960
(Using -g causes differences in generated core) is fixed.