Certain data accesses in the kernel should have volatile semantics to be correct...

Certain data accesses in the kernel should have volatile semantics to be correct and not rely on the whims of the compiler. Change queue clearing to simply catch read up to write rather than reset both to 0 to ensure sane results for queue_count and queue_empty with concurrency. Binsize may or may not increase a bit depending upon whether the output was as intended in all places; wrong stuff was already unlikely to cause any issue.

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@28909 a1c6a512-1295-4272-9138-f99709370657