rust: update to 1.54

This updates to Rust 1.54.0, which was marked stable on 29-Jul. This
also includes:
- the contents of I3a03e836b581fbddcd694fe673bcf216c4625053,
  which allows Rust to generate binaries which are instrumented for PGO.
- an explicit specification of supported LLVM targets to build, since we
  only support x86 and ARM (and their 64-bit variants)

BUG=b:194823966
TEST=CQ

Change-Id: Ie1cbd3cdc7af378f25338f7303cf2422c95136b7
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/chromiumos-overlay/+/3123545
Tested-by: George Burgess <gbiv@chromium.org>
Reviewed-by: Manoj Gupta <manojgupta@chromium.org>
22 files changed