[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/infrun.c

@@ -4958,7 +4958,7 @@ adjust_pc_after_break (struct thread_info *thread,
      we would wrongly adjust the PC to 0x08000000, since there's a
      breakpoint at PC - 1.  We'd then report a hit on B1, although
      INSN1 hadn't been de-executed yet.  Doing nothing is the correct
-     behaviour.  */
+     behavior.  */
   if (execution_direction == EXEC_REVERSE)
     return;