rust: Fix assertion failure error on oe-selftest

rust: Fix assertion failure error on oe-selftest

Fixes: thread 'main' panicked at 'assertion failed: `(left == right)`
       left: `x86_64-unknown-linux-gnu`,
       right: `x86_64-poky-linux-gnu`: Cannot obtain compiler for non-native build triple at stage 0', compile.rs:1474:13

Add correct target value for cross-compiled targets on stage1 during rust oe-selfest.

Signed-off-by: Yash Shinde <Yash.Shinde@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>