[gdb/contrib] Add codespell:ignore-begin/ignore-end (disabled)

[gdb/contrib] Add codespell:ignore-begin/ignore-end (disabled)

It would be useful to tell codespell to ignore blocks of code.

A feature ignore-multiline-regex exists, which can be used to implement this:
...
$ codespell --ignore-multiline-regex \
    'codespell:ignore-begin.*codespell:ignore-end'
...

Unfortunately there's a bug in codespell where using -w in
combination with --ignore-multiline-regex drops all newlines in the updated
file.

In absence of a fix, commit this solution disabled, to locally document the
current state of this feature.