Protect bzero call against macro expansion

Shortly, we'll be moving to defining bzero and memset in terms of
__builting_memset. To do that, we can't have macro calls to bzero in
the fallback impelmentation of memset. Normal calls to bzero are fine.
All 4 architectures that use this have their own copies of bzero, so
there's no mutual recursion issue between memset and bcopy.