ghcjs: replace integer-simple with integer-gmp

GHCJS uses integer-gmp, but cabal2nix generates a dependency list that
includes integer-simple instead. This tweaks the stage2 generator to
replace any instance of integer-simple with integer-gmp.

Things currently still work without this change (assuming
integer-simple is defined as null), as ghcjs includes integer-gmp in
its stage1 packages - so it's always available.

However, this change makes things a bit more explicit, rather than
leaving things to chance. If at any point the stage1 packages are also
split up into separate derivations, the integer-gmp package will need to
be passed along to the packages that depend on it. This change should
prevent some confusion going forward.