Move "target has RTS linker" out of configure

Currently, the testsuite is the only thing depending on the target has RTS linker information provided by ghc --info. This information is based on a whitelist of supported architectures, and can be very simply implemented as a predicate in GHC in order to provide it in ghc --info. Contrary to this, we currently determine whether target has RTS linker or not in configure, and pass it around in hadrian and in the settings file unnecessarily.

This came up in the refactor to support multiple toolchains.

I'm pushing a clean up soon which makes this much simpler (and soft unblocks some of the multiple toolchain work).