Pure luck that this ever worked at all. The optimized assembly for XOR
in RAID-5 declared did clobber registers, but did declare them as read-only.
I'm pretty sure that at least the 4 disk and possibly the 5 disk cases
did corrupt callee saved registers. The others probably got away because
they were always used in own functions (and only clobbering caller saved
registers)and only called via pointers, preventing inlining.

Some of the replacements are a bit complicated because the functions
exceed gcc's 10 asm argument limit when each input/output register needs
two arguments. Works around that by saving/restoring some of the registers
manually.

I wasn't able to test it in real-life because I don't have a RAID
setup and the RAID code didn't compile since several 2.5 releases.
I wrote some test programs that did test the XOR and they showed
no regression.

Also aligns to XMM save area to 16 bytes to save a few cycles.