ANV futex shim needs a fallback below macOS 14.4 #10

The Darwin Mesa patch implements futexes on os_sync_wait_on_address / os_sync_wake_by_address_*, which exist only from macOS 14.4. Mesa therefore builds at -mmacosx-version-min=14.4 (libdrm builds at 11.0). For macOS 11 to 14.3 we need a fallback chosen at run time, for example a hashed table of pthread mutex + condvar pairs keyed by address (no private __ulock calls). Build notes: NvGfx branch xe, gpu/userland/BUILD.md.