It turns out that the libgmp.so is provided by the `libgmp-dev` package.

The Cabal build is enormous and constitutes the majority of the build
time of the AArch64 and armv7 images. This cuts the Cabal build time
down by 60%.

See https://mail.haskell.org/pipermail/ghc-devs/2019-May/017600.html