[rust] Add assertion for LTO builds.

LTO Android builds with Rust failed in a mysterious way due to a
conflict between -Clto and -Cembed-bitcode=no. Instead, fail with an
explicit assertion.

Bug: 1069271
Change-Id: Id2a441c38539d05b7e29279ef10f160d56e31f2c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4345759


Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Commit-Queue: Adrian Taylor <adetaylor@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1118904}