Wasm runtime: add stubs for caml_create_file and caml_mount_autoload #867

Job	Run time
build (ubuntu-latest, 4.14, true, false, false)	6m 4s
build (ubuntu-latest, 5.3, true, false, false)	8m 13s
build (macos-latest, 5.3, true, false, false)	8m 50s
build (ubuntu-latest, 5.1, true, false, false)	7m 46s
build (ubuntu-latest, 5.2, false, true, false)	17m 55s
build (ubuntu-latest, 5.2, true, true, true)	18m 8s
build (windows-latest, 5.2, true, true, true)	28m 18s
build (ubuntu-latest, 5.0, true, false, false)	7m 56s
	1h 43m 10s

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