Spidermonkey is required for essential packages like polkit or Gnome/Cinnamon. However the compilation of spidermonkey has very high requirements on the system, which have even more increased with the version bump from spidermonkey 68 to 78: At least 6400MB of hard disk for spidermonkey 78 and very high ram usage due to rust. Therefore: Please consider providing a binary package, otherwise the usage of Gentoo on older systems with limited hardware resources will be unfeasible.
Is is not a official solution, but spidermonkey can be replaced by duktape. It has a much lower compile requirements. See bug #734326.
I may be wrong, but unlike firefox-bin I don't think mozilla distributes a usable pre-built libmozjs. There are very few cases where *-bin packages are built by Gentoo at the moment and it's upstream's binaries.
Spidermonkey is now available in the official Gentoo binhost repository. Don't think we can do better than that, as ionen pointed out, upstream doesn't provide recompiled package for this.
