libcore: Reexport a couple of widely-used low-level intrinsics to reduce
code bloat.

This didn't make a difference in any compile times that I saw, but it
fits what we're doing with `transmute` and seems prudent.
auto merge of #15569 : pcwalton/rust/reexport-intrinsics, r=cmr

code bloat.

This didn't make a difference in any compile times that I saw, but it
fits what we're doing with `transmute` and seems prudent.

r? @alexcrichton