MFC: r340656

Given that the idea of D15374 was to "make memmove a first class citizen",
provide a _MEMMOVE extension of _MEMCPY that deals with overlap based on
the previous bcopy(9) implementation and use the former for bcopy(9) and
memmove(9). This addresses my D15374 review comment, avoiding extra MOVs
in case of memmove(9) and trashing the stack pointer.