PhysX: Leave the mutex priority ceiling out for emscripten

Emscripten is PX_LINUX to PhysX, so the mutex constructor calls
pthread_mutexattr_setprioceiling(). Its musl has no such function, and
emscripten only defines one in the threaded pthread library, where it
does nothing but return EPERM. A build without pthreads links the stub
library instead, which has no definition, so every application linking
the bundled PhysX for WebAssembly without threads failed to link.

PhysX 4.1 never made that call, so this came with the update to
PhysX 5. Skip it for emscripten altogether, since it cannot set a
ceiling with or without threads, and store the change as patch 0018.
pthread_mutexattr_setprotocol() comes from musl itself, which the single
threaded libc includes, so it stays.

Amends 7533abcc02b7f9bd7953203c489ee86d55e8c909

Fixes: QTBUG-151206
Assisted-by: Claude Code
Change-Id: Ib68e93286da0e8b2558a94dbbc6c357d559d9eb7
Reviewed-by: Paul Olav Tvete <paul.tvete@qt.io>