Add support for WASM backend #3234

Job	Run time
nix-build	3m 29s
unit-tests	10s
runghc	1m 3s
tests-benchmarks	1m 1s
shellFor-multiple-package	1m 19s
shellFor-hoogle	1m 12s
shellFor-not-depends	1m 19s
cabal-doExactConfig	54s
build-extra-hackages	13s
multi-target	2m 0s
maintainer-scripts	8s
cabal	54s
shellFor-single-package	30s
plan-extra-hackages	13s
hydra-without-remote-builders-ghc967	3m 11s
hydra-without-remote-builders-ghc9102	3m 1s
docs	2m 19s
hydra-ifdLevel-2	9m 24s
hix	32s
hydra-ifdLevel-3	30m 18s
haskell-nix-roots-do-not-require-IFDs	24s
hix-cabal	15s
hydra-ifdLevel-0-and-1	3m 28s
check-nix-tools	17s
check-materialization-concurrency	24s
closure-size	7s
check-path-support	20s
update-docs	8s
	1h 8m 33s

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