It's not possible to specify multiple `-with-rtsopts` flags

Motivation

Currently -with-rtsopts can only be specified once, repeat uses will clobber each other. The result of this is that it's cumbersome to, for example, conditionally specify/add RTS flags in cabal files (for example, adding additional compile time RTS options for profiling builds).

Proposal

There are two possible ways to fix this issue:

1. Simply change -with-rtsopts to concatenate all the options from each flag
2. Add a new flags -add-rtsopt or something along those lines which concatenates all its invocations together. This approach has several options:
   1. Extend the the last preceding -with-rtsopts with the new added options. (Confusing if a later -with-rtsopts clobbers this)
   2. Make -with-rtsopts and -add-rtsopt mutually exclusive
   3. Extend the final -with-rtsopts with these new options

Options 1 has the benefit of not adding a new flag and working as many people (anecdotally) already expect the flag to work, but could theoretically break existing uses of -with-rtsopts. Option 2.1 I think is to confusing to really consider. Option 2.2 is unambiguous, but also not very convenient. Option 2.3 plays nicely with (possible) existing uses of -with-rtsopts clobbering behaviour without making the behaviour of -add-rtsopt unnecessarily confusing.

Personally I tend towards options 1 or 2.3, but I don't have a very strong opinion on the bikeshed colour. It would be useful for people to chime in which option makes the most sense.

It's unclear if anyone is even using/relying the current behaviour of -with-rtsopts, so I'll create a comment for each option and people can thumbs up/down each option to poll opinion on the bikeshed colour.