Uses/cabal.mk: Add support for LTO feature.

Haskell code benefits greatly from splitting functions into distinct ELF
sections and then letting the linker to garbage-collect them.

devel/hs-haskell-language-server port goes from 277MiB to 176MiB in size.
net-p2p/cardano-node ports goes 302MiB -> 76.2MiB

Relevant blog post: http://brandon.si/code/linking-smaller-haskell-binaries/

Reported by: Greg Steuck <greg@nest.cx>