arch: gate x86_64 syscall_entry asm out of cfg(test)

What:
src/arch/x86_64/syscall.rs: the global_asm! defining `syscall_entry` was
gated `#[cfg(not(fuzzing))]`; widen to `#[cfg(not(any(fuzzing, test)))]`
so it matches its four sibling asm blocks and is excluded under
`cargo test`.

Why:
Under `cargo test` the crate already compiles a host shim,
`#[cfg(any(fuzzing, test))] mod fuzz_asm_stubs`, which provides a
`#[no_mangle] syscall_entry` stub. The real global_asm being gated only
`not(fuzzing)` meant it stayed compiled under test too, so `syscall_entry`
had two definitions. On Mach-O the leading-underscore symbol convention
separated them (`syscall_entry` asm vs `_syscall_entry` stub), so
`cargo test --target x86_64-apple-darwin` linked fine and the bug stayed
hidden. On ELF there is no underscore: the first CI run on a Linux runner
failed to link the lib-test binary with
`rust-lld: error: duplicate symbol: syscall_entry`.

The four sibling asm blocks (context_switch, timer_isr_stub,
yield_save_and_switch, gdt_reload_segments) were already
`not(any(fuzzing, test))`; this was the lone block missing `test`, which
is why the linker reported exactly one duplicate.

Behaviorally inert for the shipping kernel: in a real build (not test,
not fuzzing) the cfg still evaluates true and the asm is compiled,
identical to before. Only the test build changes -- it now defers to the
existing no-op stub, exactly as the other four symbols already do.

Out of scope:
No change to the syscall ABI, the asm body, or any SAFETY contract; only
the cfg predicate widens.

Verification:
  - make check-all -> all three architectures build cleanly
  - cargo test --lib --target x86_64-apple-darwin -> 892 passed; 0 failed
  - the ELF lib-test link (the original CI failure) is confirmed by the
    next CI run

Staged files:
  src/arch/x86_64/syscall.rs

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: riccajason

src/arch/x86_64/syscall.rs

@@ -132,7 +132,7 @@ pub unsafe fn init() {
 //   RBX, RBP, R12-R15 (callee-saved)
 //   SyscallFrame      ← RSP points here when handler is called
 
-#[cfg(not(fuzzing))]
+#[cfg(not(any(fuzzing, test)))]
 core::arch::global_asm!(
     ".global syscall_entry",
     "syscall_entry:",