Generate and/or use PGO profiles in the Rust compiler.

Introduce USE flags to either build Rust with PGO instrumentation, or
use PGO profiles to build an optimized Rust.

BUG=b:234046818
TEST=Use for PGO in Rust 1.62.1

Change-Id: I82372e405ef56f9e77f76d30ab3610ae804e2699
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/chromiumos-overlay/+/3762719
Reviewed-by: George Burgess <gbiv@chromium.org>
Tested-by: Michael Benfield <mbenfield@google.com>
2 files changed