[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

gdbserver/server.cc

@@ -1544,7 +1544,7 @@ parse_debug_options (const char *options)
   gdb_assert (options != nullptr);
 
   /* Empty options means the "default" set.  This exists mostly for
-     backwards compatibility with gdbserver's legacy behaviour.  */
+     backwards compatibility with gdbserver's legacy behavior.  */
   if (*options == '\0')
     options = "+threads";