Be sure to register a new mutex owner _before_ waking it. Won't be an issue now but...

Be sure to register a new mutex owner _before_ waking it. Won't be an issue now but would be with mutex recursion on one used for > 1 core where ownership transfer and cs entry/recursion are allowed to run in parallel (by design). TODO: Add true exchange to wakeup_thread but that's not really important for the time being.

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@15251 a1c6a512-1295-4272-9138-f99709370657