Build a threaded stage 1 if the bootstrapping GHC supports it.

GHC's build system assumes that the bootstrapping GHC might not have the threaded RTS way available, so it builds stage 1 without -threaded. This is unfortunate for cross compilers, since this means they don't work with -j.

This patch has configure check whether or not the bootstrapping GHC supplies any threaded RTS ways, and builds stage 1 with -threaded if it does.

I'm not an autoconf expert, and I've only just now got my head around Hadrian, so if there's a better way to achieve this behavior I'm all ears.

Fixes #18024 (closed).