Edit: actually, looks like I'm wrong emghost appears to know more about this than me.
https://twitter.com/StefanKarpinski/status/12929837172128931...
Since then, GCC has stepped up with this out-of-tree build, but it's still not 100% to my knowledge.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96168
Edit: actually, looks like I'm wrong emghost appears to know more about this than me.