More robust Wasm support

[dhess]

Specifically:

• fix regression in fix: make Wasm upstream overrides dependent on arch(wasm32) #1191. (Fixed by chore: get the wasm32 build working again #1195)
• Make sure we're always using -O2 for Wasm builds, since it's not really a viable development/debugging target, anyway.
• Get it working on macOS, probably via https://github.com/minoki/ghc-docker
• Run benchmarks on Wasm target (probably requires migrating to tasty-bench, see feat: build & run primer-benchmark for Wasm32 targets #1190)