lib: move rbtree code

lib: move rbtree code

Build this code into an archive, which results in not linking it into
x86 final binaries. This saves about 1.5k of dead code.

While moving the source file, take the opportunity and drop the
pointless EXPORT_SYMBOL() and an instance of trailing whitespace.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Wei Liu <wl@xen.org>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>