Add support for WASM backend #3334

Job	Run time
nix-build	5m 12s
tests-benchmarks	57s
shellFor-single-package	24s
unit-tests	10s
shellFor-multiple-package	1m 51s
cabal-doExactConfig	1m 3s
shellFor-hoogle	16s
shellFor-not-depends	22s
maintainer-scripts	10s
multi-target	49s
build-extra-hackages	12s
plan-extra-hackages	11s
runghc	13s
cabal	14s
check-nix-tools	17s
hix	36s
hydra-ifdLevel-3	30m 17s
hydra-without-remote-builders-ghc967	13m 29s
update-docs	8s
docs	14m 9s
check-path-support	16s
hydra-without-remote-builders-ghc9102	12m 35s
closure-size	8s
hydra-ifdLevel-0-and-1	3m 9s
hydra-ifdLevel-2	5m 30s
check-materialization-concurrency	23s
hix-cabal	2m 48s
haskell-nix-roots-do-not-require-IFDs	25s
	1h 36m 14s

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