[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

gdbsupport/parallel-for.h

@@ -136,7 +136,7 @@ parallel_for_each (unsigned n, RandomIt first, RandomIt last,
 }
 
 /* A sequential drop-in replacement of parallel_for_each.  This can be useful
-   when debugging multi-threading behaviour, and you want to limit
+   when debugging multi-threading behavior, and you want to limit
    multi-threading in a fine-grained way.  */
 
 template<class RandomIt, class RangeFunction>