[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/annotate.h

@@ -112,7 +112,7 @@ struct annotate_arg_emitter
    are on, we only sometimes print the annotation, and only sometimes
    update the current symtab and line.  However, this particular annotation
    has behaved this way for some time, and front ends that still use
-   annotations now depend on this behaviour.  */
+   annotations now depend on this behavior.  */
 extern bool annotate_source_line (struct symtab *s, int line,
 				  int mid_statement, CORE_ADDR pc);