Add support for WASM backend #3318

Job	Run time
unit-tests	12s
nix-build	6h 0m 18s
runghc	6h 0m 17s
tests-benchmarks	11s
cabal-doExactConfig	46m 58s
multi-target	6h 0m 16s
shellFor-single-package	1h 25m 32s
shellFor-multiple-package	2h 13m 23s
cabal	13s
shellFor-hoogle	1h 30m 40s
maintainer-scripts	9s
plan-extra-hackages	10s
shellFor-not-depends	19s
build-extra-hackages	15s
hydra-ifdLevel-0-and-1	2m 46s
hydra-ifdLevel-2	3m 40s
hydra-ifdLevel-3	30m 17s
update-docs	9s
check-path-support	12s
docs	6h 0m 17s
haskell-nix-roots-do-not-require-IFDs	19s
closure-size	8s
check-materialization-concurrency	22s
hydra-without-remote-builders-ghc967	11s
hydra-without-remote-builders-ghc9102	24s
hix-cabal	2m 48s
check-nix-tools	14s
hix	1m 24s
	1d 6h 42m 4s

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