memmove: Don't call memcpy if regions overlap

memmove() unconditionally calls memcpy() if "dst" < "src". For
example, in the code below, memmove() would end up calling memcpy(),
even though the regions of memory overlap.

int main() {
  char buf3[0x800];
  char *dst  = &buf3[1];
  char *src = &buf3[0x400];
  memset(buf3, 0, sizeof(buf3));
  memmove(dst, src, 0x400);
  printf("1: %s\n", buf3);
  return 0;

Calling memcpy() on overlaping regions only works if you assume
that memcpy() copies from start to finish. On some architectures,
it's more efficient to call memcpy() from finish to start.

This is also triggering a failure in some of my code.

More reading:
* (comment 31)

Change-Id: I65a51ae3a52dd4af335fe5c278056b8c2cbd8948
1 file changed