[roll] Roll fuchsia [zircon][rust] Verify Rust syscall signatures via Zither

Implement compile-time signature verification for Rust kernel syscall
handlers against the Zither FIDL schema.

Extend Zither's kernel backend to emit syscall_sigs.rs, containing
Rust function pointer type aliases (fn(...) -> ...) derived directly
from FIDL syscall definitions. Define this output as a standalone kernel
Rust crate (syscall-signatures) in //zircon/vdso.

Introduce the #[syscall] procedural macro in syscalls-macro. The
macro enforces that handler function names begin with sys_, wraps
them in an exported extern "C" ABI shim, and emits an anonymous
constant type assertion comparing the handler's signature against the
Zither-generated signature in syscall-signatures. Migrate all test
syscall handlers to use #[syscall].

Original-Original-Bug: 527587271
Test: fx build and fx run-boot-test kernel-unittests-boot-test
Original-Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1681546
Original-Original-Revision: d8e6f46fbad96e133d3d7992c42012f90846f1af
GitOrigin-RevId: cffd1bf6154d2db9040252e51e35bf821cda3a66
Change-Id: I49952c7a562d8e5b15600f6d6bd84ddb08a898e2
Reviewed-on: https://fuchsia-review.googlesource.com/c/integration/+/1686077
Cr-Commit-Position: refs/heads/main@{#195840}