[gdb/contrib] Add two rules in common-misspellings.txt

Eli mentioned [1] that given that we use US English spelling in our
documentation, we should use "behavior" instead of "behaviour".

In wikipedia-common-misspellings.txt there's a rule:
...
behavour->behavior, behaviour
...
which leaves this as a choice.

Add an overriding rule to hardcode the choice to common-misspellings.txt:
...
behavour->behavior
...
and add a rule to rewrite behaviour into behavior:
...
behaviour->behavior
...
and re-run spellcheck.sh on gdb*.

Tested on x86_64-linux.

[1] https://sourceware.org/pipermail/gdb-patches/2024-November/213371.html

gdb/break-cond-parse.c

@@ -368,7 +368,7 @@ parse_all_tokens (const char *str)
      which exists for backwards compatibility reasons.  Traditionally this
      flag, if it occurred immediately after the 'if' condition, would be
      treated as part of the 'if' condition.  When the breakpoint condition
-     parsing code was rewritten, this behaviour was retained.  */
+     parsing code was rewritten, this behavior was retained.  */
   gdb_assert (backward_results.empty ()
 	      || (forward_results.back ().get_type ()
 		  == token::type::CONDITION));