Created attachment 519686 [details]
Issue is reproducible. Remerge of kernel gentoo-sources did not change the issue.
Compilation works <4.15.0.
Created attachment 519688 [details]
It's failing because of CONFIG_DEBUG_INFO_SPLIT in the kernel config. try disabling that.
I'm currently working on linux-mod.eclass and can look at the issue a bit deeper, seems it needs some patching or sandbox massage.
this bug should depend on https://bugs.gentoo.org/653286
fixed in eclass