On macOS pthread id is a pointer to struct _opaque_pthread_t type,
requires explicit cast to ulint which in turn is size_t;
Was failing with Clang 9.1.0 Debug build on macOS 10.13.4:

sync0policy.h:53:4: error: cannot initialize a member subobject of type 'ulint'
(aka 'unsigned long') with an rvalue of type 'os_thread_id_t' (aka '_opaque_pthread_t *')
m_thread_id(os_thread_id_t(ULINT_UNDEFINED))
                        ^           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
sync0policy.h:79:4: error: cannot initialize a parameter of type 'int64' (aka 'long long') with
an rvalue of type 'os_thread_id_t' (aka '_opaque_pthread_t *')
my_atomic_storelint(&m_thread_id, os_thread_get_curr_id());