add GHC 8.10.3

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The Alpine 32bit bindist is (still?) missing.

I build alpine bindists manually, usually. These are important because they are used for static builds (including ghcup itself).

Not sure if there are automated ones already @bgamari

I didn't realize there was any demand for 32-bit Alpine. We currently don't build such bindists in CI. Do we really think it would be worth doing so? If so please do add a release job in CI.

To clarify, we currently do offer 64-bit Alpine builds (built against integer-simple). See https://downloads.haskell.org/ghc/8.10.3/ghc-8.10.3-x86_64-alpine3.10-linux-integer-simple.tar.xz.

merged