After some digging, I figured that I needed gcc6.

There is a line at the docs on https://github.com/rbenv/ruby-build/wiki#arch-linux, but not much information on how to use it:

If needed, install gcc6 from the AUR.

Here is how to do it:

1. Install gcc6 from AUR (note: takes a long time to compile)

2. Install with:

RUBY_CONFIGURE_OPTS="--with-openssl-dir=/usr/lib/openssl-1.1 --with-gcc=gcc-6" asdf install ruby 3.1.0