[gdb/contrib] Add two words to common-misspellings.txt

While reviewing changes generated by spellcheck.sh for directory sim, I
noticed two more misspellings:
...
arrithemetic->arithmetic
electricaly->electrically
...

Add them to common-misspellings.txt, and fix them in directory sim.

Tested by rebuilding on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>