Gentoo's Bugzilla –
Attachment 409488 Details for
Bug 508502app-emulation/virtualbox-modules - x86_64-pc-linux-gnux32-ld: Relocatable linking with relocations from format elf64-x86-64 (.../work/vboxdrv/linux/SUPDrv-linux.o) to format elf32-x86-64 (.../work/vboxdrv/vboxdrv.o) is not supported