[gdb/contrib] Drop two words from codespell-ignore-words.txt

[gdb/contrib] Drop two words from codespell-ignore-words.txt

Tom Tromey mentioned [1] that the words "invokable" and "useable"
present in codespell-ignore-words.txt should be dropped.

Do so and fix the following typos:
...
$ codespell --config gdbsupport/setup.cfg gdbsupport
gdbsupport/common-debug.h:218: invokable ==> invocable
gdbsupport/event-loop.cc:84: useable ==> usable
...

Approved-By: Tom Tromey <tom@tromey.com>
[1] https://sourceware.org/pipermail/gdb-patches/2025-March/216584.html