The canonical form to discard all sections not mentioned earlier in
the script is
/DISCARD/ : { *(*) }
not
/DISCARD/ : { *(.*) }
".*" happens to work with the usual section names starting with a dot,
but let's not promote something not quite right.
there's no reason why the tree-based variant can't always be used
when sorting is required, it merely needs to also support filename
sorting and have a fast path for insertion at end (aka rightmost tree
leaf).
The filename sorting isn't tested anywhere and the only scripttempl
that uses it is avr (for 'SORT(*)(.ctors)'), and I believe even there it
was a mistake. Either way, this adds a testcase for filename sorting as
well.
Then the non-BST based sorting can be simplified to only support
the fast case of no sorting required at all (at the same time renaming
the two variants to _sort and _nosort).
