(fold_name): If assigning_to_dot is true, object immediately to
an undefined symbol.
(exp_fold_tree): Set and clear assigning_to_dot around the
recursive call to exp_fold_tree to process the right-hand side
of an assignment to the location counter.
testsuite:
* ld-scripts/align.exp: Rename existing "ALIGN" test to "align1".
Add dump tests "align2a", "align2b", "align2c".
* ld-scripts/align2.t, ld-scripts/align2a.s, ld-scripts/align2a.d
* ld-scripts/align2b.s, ld-scripts/align2b.d
* ld-scripts/align2c.s, ld-scripts/align2c.d: New files.
