PSA: delete gnulib/config.cache and/or gdb/config.cache

Hello,

I have just pushed
https://gnutoolchain-gerrit.osci.io/r/c/binutils-gdb/+/681, which
updates gnulib to today's upstream version.

Unfortunately, it seems that you have to delete gnulib/config.cache
and/or gdb/config.cache for rebuilds to work correctly, so this is a
PSA to do that after updating (or, of course, you can delete your
entire build output directory).

Christian