The assembly code used to callback into Open Firmware client
interface in 32 bits mode used to backup the stack pointer in
the SPRG2 register.

That upsets Apple's implementation of Open Firmware significantly
and maybe others, causing them to crash in _some_ operations,
apparently the trigger is to cause a segment or hash table
fault, typically happens when letting that code initialize the
second display.

This patch fixes it, along with other cleanups of that asm code,
it did unnecessary register restores and backing up the stack
pointer is actually useless anyway.