[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/gdbthread.h

@@ -216,7 +216,7 @@ struct thread_suspend_state
      - If the thread is running, then this field has its value removed by
        calling stop_pc.reset() (see thread_info::set_executing()).
        Attempting to read a std::optional with no value is undefined
-       behaviour and will trigger an assertion error when _GLIBCXX_DEBUG is
+       behavior and will trigger an assertion error when _GLIBCXX_DEBUG is
        defined, which should make error easier to track down.  */
   std::optional<CORE_ADDR> stop_pc;
 };