Linking with lld in Haskell builds

10x faster than ld in the linkin steps, 3x faster than gold.

Use cabal configure --with-gcc=clang --with-ld=clang, then cabal build.

See the .cabal file for what options are needed.

Bugs

cabal bugs that prevent using lld in the library linking step:

• cabal does not respect ghc-options for -pgmc, -pgml etc

You can't currently use lld correctly in nix because of this bug:

• LLD linker does not have required nixpkgs wrapper script and flags