The highest-order bit of the bootloader cookie is 1, with the result that
the 32-bit cookie can be sign-extended on its way out of the loader and
through Open Firmware. If sign-extended, the in-kernel check of its value
would fail on 64-bit systems, resulting in a mountroot prompt. Solve this
by telling the kernel to ignore the high-order bits.

PR: kern/224437
Submitted by: Gustavo Romero