Remove RTS hack for configuring

Closed John Ericson requested to merge wip/remove-rts-configure-hack into master

See the brand new Note [Undefined symbols in the RTS] for additional details.

The note mentions

GHC already does -undefined dynamic_lookup, so we just do that for now, but we might try to get more precise with -U in the future.

Indeed I plan on doing that once the external symbols logic is moved to the configure script (the next 1 or 2 MRs after this). That way this can be done programmatically as opposed to with more combinatorially-exploding cabal file boilerplate.

That will help us lock down the RTS ABI better in the long term. (In the medium term it doesn't achieved much because the RTS is still linked with -undefined dynamic_lookup so there is no enforcement, an infelicity that predates this MR, but that should also be improved.)

Progress towards #17191

Edited by John Ericson

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