Add -rtsopts to unison ghc-options

[mitchellwrosen]

This PR just adds -rtsopts to the unison executable, so a greater number of runtime knobs can be tweaked with command-line options

[aryairani]

We'll have to do this again after #1603. Also do you know if and how it affects build times?

[aryairani]

Also I think stack build --profile does this, along with building the libraries with the right settings? or am I mixed up?

[mitchellwrosen]

It doesn't affect build times, it's just whether the resulting executable allows to to tweak things (honestly, I'm not really sure why you wouldn't want to set this 🤷)

About stack build --profile - I'm not sure if that sets -rtsopts too; at least in GHC they're distinct (-rtsopts and -prof)

[aryairani]

If -rtsopts is required to be able to supply +RTS ... options to the built executable's command line, then --profile definitely enables at least that; if it isn't, then I dunno.

[mitchellwrosen]

Not all +RTS opts are disabled without -rtsopts, only "most" according to GHC. -s (print stats) and -p (profile? or something?) both seem to work, but things like tweaking the nursery size (-A) do not