[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/corelow.c

@@ -1523,7 +1523,7 @@ core_target::xfer_partial (enum target_object object, const char *annex,
    exactly lively, are they?  On the other hand, if we don't claim
    that each & every one is alive, then we don't get any of them
    to appear in an "info thread" command, which is quite a useful
-   behaviour.
+   behavior.
  */
 bool
 core_target::thread_alive (ptid_t ptid)