[roll] Roll fuchsia [kernel] Make Rust-based link unconditional

This removes the conditionals testing the `enable_rust_in_zircon`
GN build argument around the build support and kernel unittests
for Rust code in the kernel.  The vmzircon executable is now
always created by the Rust compiler driver, but that's just a
different driver invoking the same underlying linker and not
adding anything else new to the link compared to linking the same
vmziiron with the same linker via the C++ compiler driver.

The vmzircon.with-tests executable now unconditionally contains a
little actual Rust code, for the kernel/tests/rust unit tests.

The `enable_rust_in_zircon` GN build argument still exists and
still defaults to false.  This can be used to gate switching
load-bearing code to involve Rust.

Original-Original-Fixed: 518907861
Original-Original-Bug: 518528981
Run-All-Tests: true
Original-Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1623294
Original-Original-Revision: b6a254435ffbf2fd26408428e33ab4a55b33cb1f
GitOrigin-RevId: ddbaa77f119482ea26e42d428f998fe19f287046
Change-Id: If231b90f73b43c4f74127e6e305c5774374eac33
Reviewed-on: https://fuchsia-review.googlesource.com/c/integration/+/1641761
Cr-Commit-Position: refs/heads/main@{#194296}