kernel - Optimize bcopy, bzero, memset

kernel - Optimize bcopy, bzero, memset

* Use __builtin_memset() for bzero() and __builtin_memmove()
  for bcopy().

  - Must use _bcopy in a few places where GCC complains about
    structural punning.  Even casting doesn't help.

  - GCC's __builtin_memset() and __builtin_memmove() has a side
    effect where it assumes that the pointer arguments cannot be
    NULL.  In fact, they can be NULL when the byte count is 0.
    This assumption by GCC causes later unrelated conditionals
    on the pointers against NULL to be improperly optimized-out.

    We had to fix one place where this blew the system up.

* Implement memset() in assembly (remove from libkern).

* Implement memmove() in assembly (remove from libkern).