This is an auto-filled bug because sys-process/runit calls ar directly.
The issue was originally discovered on arm64, but it may be reproducible on other arches as well.
If you think that a different summary clarifies the issue better, feel free to change it.
Attached build log and emerge --info.
If you think it doesn't make sense fix these type of issues, I'd like to point out that won't be possible use a different AR implementation (like llvm-ar) by setting the AR variable. So this issue has been reproduced by setting the AR variable to aarch64-unknown-linux-gnu-ar and by removing the /usr/bin/ar binary.
Created attachment 637068 [details]
build log and emerge --info
the same happens for ranlib as showed in the build log.
Created attachment 639104 [details]
make ar, ranlib configurable
Here's a patch from my overlay against the runit 2.1.2 source that makes both ar and ranlib configurable by adding conf-ar and conf-ranlib files, similar to its existing conf-cc and conf-ld ones.
With this fixing the ebuild should be fairly trivial.