add volatile copy/copy_nonoverlapping/set

This exposes volatile versions of the memset/memmove/memcpy intrinsics.

The volatile parameter must be constant, so this can't simply be a
parameter to our intrinsics.
auto merge of #13693 : thestinger/rust/mem, r=alexcrichton

This exposes volatile versions of the memset/memmove/memcpy intrinsics.

The volatile parameter must be constant, so this can't simply be a
parameter to our intrinsics.