[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/extension.h

@@ -177,7 +177,7 @@ enum ext_lang_rc
 
   /* There was an error (e.g., Python error while printing a value).
      When an error occurs no further extension languages are tried.
-     This is to preserve existing behaviour, and because it's convenient
+     This is to preserve existing behavior, and because it's convenient
      for Python developers.
      Note: This is different than encountering a memory error trying to read
      a value for pretty-printing.  Here we're referring to, e.g., programming