Gentoo already has it, however the latest ebuild is still masked, so one would need to put "sys-kernel/linux-firmware ~amd64" inside a file in /etc/portage/package.accept_keywords, or better yet, always run the git version, using * instead of ~amd64.

Apart from that, it's necessary to "sudo emaint sync -A && sudo emerge -av sys-kernel/linux-firmware", while checking that the correct files are included in the savedconfig file if using it. After that, rebuild the kernel or the initramfs and reboot.

The 6.4.6 kernel has mitigations, but the arch “linux” package is still at 6.4.5.