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 #15960
(Using -g causes differences in generated core) is fixed.
gold is known to speed up GHC builds considerably, saving up to many seconds
when linking large Haskell applications, so this patch configures all
non-binary GHC derivations to use gold.