I tried to install openrct2 and it searched for some videocore specific stuff at /opt/vc/include. My package is: media-libs/raspberrypi-userland-0_pre20201022 . Installing the userland using git (not the 9999 version) as in https://wiki.gentoo.org/wiki/Raspberry_Pi/ARM64_in_place_installation#Installing_Raspberry_Pi_userland might work, too.
The following steps helped me to get it emerged (I didn't write a diary, so hopefully that were the steps):
ln -s /usr/include/vcinclude include
ln -s /usr/include/interface/