elf: Simplify software TM implementation in _dl_find_object

With the current set of fences, the version update at the start
of the TM write operation is redundant, and the version update
at the end does not need to use an atomic read-modify-write
operation.

Also use relaxed MO stores during the dlclose update, and skip any
version changes there.

Suggested-by: Szabolcs Nagy <szabolcs.nagy@arm.com>
Reviewed-by: Szabolcs Nagy <szabolcs.nagy@arm.com>