microcode.cpio gets generated by sys-apps/microcode-data and it would be nice if an initrd line got added automatically when the file is in /boot.
Ask for it upstream.
Honestly most distros are combining their initrd with the microcode initrd and calling it a day. The path forward would be to add this support to genkernel or use dracut which I presume supports that already.
*** Bug 587422 has been marked as a duplicate of this bug. ***