opts: improve option suggestion

opts: improve option suggestion

In case where we have 2 equally good candidates like
-ftrivial-auto-var-init=
-Wtrivial-auto-var-init

for -ftrivial-auto-var-init, we should take the candidate that
has a difference in trailing sign symbol.

	PR driver/105564

gcc/ChangeLog:

	* spellcheck.cc (test_find_closest_string): Add new test.
	* spellcheck.h (class best_match): Prefer a difference in
	trailing sign symbol.