[roll] Roll fuchsia [zx] Fix rustflags so GN correctly parses the target for Zircon kernel crates

When merging rust-project.json files, Zircon kernel crates did not
specify a 'target' key because they were built using the custom target
specification '--target=kernel' (separated by '=').

In GN's upstream `rust_project_writer.cc`, the compiler target is
extracted by searching for the '--target' argument. However, the
GN parser only handles "--target <value>" (separated by a space)
and does not check for "--target=<value>".

To fix this at the root cause, this change updates the Zircon kernel
build to pass "--target" and "kernel" as separate arguments in
zircon/kernel/switch/BUILD.gn. This allows GN to correctly parse
"kernel" as the target and populate `"target": "kernel"` in the
exported rust-project.json.

Original-Original-Bug: 522275056
Original-Original-Fixed: 521448897
Test: bazel_rust_analyzer_utils_test.py
Original-Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1648157
Original-Original-Revision: 3f655f8a4e87853460f021bcb0bf43eee7ee2584
GitOrigin-RevId: 25376e25c0f96352612e19b4fcba46899aa91d09
Change-Id: Id8dfdbf46d55b6522b2eedd36b8fa6c2276c8191
Reviewed-on: https://fuchsia-review.googlesource.com/c/integration/+/1649198
Cr-Commit-Position: refs/heads/main@{#194557}