Revert the following patch brought in by "Merge pull request #753 from shinnok/10.3-macfixes"

It caused compile failures:

commit 6620fbd6
Author: Teodor Mircea Ionita <teodor@mariadb.org>
Date:   Thu May 10 12:23:35 2018 +0300

    MDEV-15778: On macOS pthread_t is opaque, requires explicit cast

    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());

storage/innobase/include/sync0policy.h

@@ -50,7 +50,7 @@ class MutexDebug {
 			m_mutex(),
 			m_filename(),
 			m_line(),
-			m_thread_id(ULINT_UNDEFINED)
+			m_thread_id(os_thread_id_t(ULINT_UNDEFINED))
 		{
 			/* No op */
 		}
@@ -76,7 +76,7 @@ class MutexDebug {
 		{
 			m_mutex = mutex;
 
-			my_atomic_storelint(&m_thread_id, reinterpret_cast<ulint>(os_thread_get_curr_id()));
+			my_atomic_storelint(&m_thread_id, os_thread_get_curr_id());
 
 			m_filename = filename;