Fix the wrong EmulateByNameResult::NotSupported in syscall shim

[ghost]

Without the change, the newly added test will fail with:

-thread 'main' panicked at 'unsupported Miri functionality: can't execute syscall with ID 0', $DIR/unsupported_syscall.rs:10:9
+thread 'main' panicked at 'unsupported Miri functionality: can't call foreign function: syscall', $DIR/unsupported_syscall.rs:10:9
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

cc #1818 (comment)

[RalfJung]

Thanks. :)
@bors r+

[bors]

📌 Commit 4f3718e has been approved by RalfJung

[bors]

⌛ Testing commit 4f3718e with merge d39f0c6...

[bors]

☀️ Test successful - checks-actions
Approved by: RalfJung
Pushing d39f0c6 to master...