[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/language.h

@@ -788,7 +788,7 @@ struct symbol *
 					    const char *name);
 
 
-/* These macros define the behaviour of the expression 
+/* These macros define the behavior of the expression 
    evaluator.  */
 
 /* Should we range check values against the domain of their type?  */