[rust] Target triples for 32-bit ARM

We inform the Rust compiler, rustc, about the intended target using a
"target triple" which is a string like "aarch64-unknown-linux-gnueabi".
This string was wrong for 32-bit ARM, as it should actually end simply
"-gnu". Give the correct target triple, and be more specific about float
ABIs.

Bug: 1445943
Change-Id: I0cdb54d996f05ed93799ce1093e6bd3b6374da64
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4538801
Reviewed-by: danakj <danakj@chromium.org>
Commit-Queue: Adrian Taylor <adetaylor@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1145422}