ARCS_LOCK_PAD or padding and aligning of mutex structures

Hi Kip,

I wondered if one shouldn't use CACHE_LINE_SIZE for ARCS_LOCK_PAD
instead of hardcoding 128? And maybe even use it for aligning the
struct, see http://fxr.watson.org/fxr/source/vm/vm_page.h#L176 .

 - Marius