Allow RTS to be reinstalled

This is the natural companion to #19896 (closed). The other libraries are hopefully most the way there already.

With #17191 pretty close, we can start thinking about the next steps:

• Don't bake decisions about libraries in rts.cabal. Either we extend Cabal so that flags can be determined by configure in the rts.buildinfo, or we rip out the flags (oh well) and rewrite all the logic (not just the detection) in the RTS configure script output the underlying cc-options, ld-options, etc. in rts.buildinfo.

• What can build the RTS? Does Cabal know how to build cmm-sources, or just install them? I think it wouldn't be too hard to hack up part of the make build system

• Split settings files: Some of the options in there are about how the RTS is built (e.g. is it unregisterized) it would be unsafe for a separately-configure RTS and settings file to disagree, so these settings should be installed as part of the RTS. @hsyl20 I am tempted to undo part of 085983e6 at that point, because if we are already planning on configuring and shipping RTS the derived constants setting file is clearer than the the big define in the C header.