[gdb/contrib] Add words.sh script

[gdb/contrib] Add words.sh script

Add a script that takes a list of files as arguments and output a list of
words from the C comments with their frequencies.

For:
...
$ ./gdb/contrib/words.sh $(find gdb -type f -name "*.c" -o -name "*.h")
...
it generates a list of ~15000 words prefixed with frequency.

This could be used to generate a dictionary that is kept as part of the
sources, against which new code can be checked, generating a warning or
error.  The hope is that misspellings would trigger this frequently, and rare
words rarely, otherwise the burden of updating the dictionary would be too
much.

And for:
...
$ ./gdb/contrib/words.sh -f 1 $(find gdb -type f -name "*.c" -o -name "*.h")
...
it generates a list of ~5000 words with frequency 1.

This can be used to scan for misspellings manually.

Change-Id: I7b119c9a4519cdbf62a3243d1df2927c80813e8b