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: I1d869c60ce925c23aed7ae43ba918e965f64fe5e
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/chromiumos-overlay/+/3831380
Tested-by: Michael Benfield <mbenfield@google.com>
Reviewed-by: George Burgess <gbiv@chromium.org>
2 files changed