Use PAUSE instruction inside spinloop if it is available.

The patch was originally developed by Mikael Ronstrom <mikael@mysql.com>
and can be found here:
http://bazaar.launchpad.net/%7Emysql/mysql-server/mysql-5.4/revision/2768
http://bazaar.launchpad.net/%7Emysql/mysql-server/mysql-5.4/revision/2771
http://bazaar.launchpad.net/%7Emysql/mysql-server/mysql-5.4/revision/2772
http://bazaar.launchpad.net/%7Emysql/mysql-server/mysql-5.4/revision/2774
http://bazaar.launchpad.net/%7Emysql/mysql-server/mysql-5.4/revision/2777
http://bazaar.launchpad.net/%7Emysql/mysql-server/mysql-5.4/revision/2799
http://bazaar.launchpad.net/%7Emysql/mysql-server/mysql-5.4/revision/2800

Approved by:	Heikki (rb://137)