[starnix] Use lutex on Fuchsia

This allows fuchsia component that use remote binder to use libfmq.

Bug: 297443970, 297431411

Change-Id: If2512194bf8f8389be5fab1b1a8409766f8e047b
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/android.googlesource.com/platform/system/libfmq/+/895758
Reviewed-by: Adam Barth <abarth@google.com>