rust: cherrypick outline atomic patches

This CL cherrypicks upstream work in LLVM that allows compiler-rt to
have out-of-line implementations of aarch64 atomics.

BUG=chromium:1180873
TEST=CQ & memd tests

Change-Id: Ib6190492907f187c94659e4cff10fd551b885535
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/chromiumos-overlay/+/2749874
Reviewed-by: George Burgess <gbiv@chromium.org>
Tested-by: George Burgess <gbiv@chromium.org>
2 files changed