[dif] Add a comment explaining how to drop an error on the ground
[dif] Add a comment explaining how to drop an error on the ground

See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=25509, which
necessitates doing this in a non-standard, but still portable, manner.

Signed-off-by: Miguel Young de la Sota <71b8e7f4945fd97b98544cf897992af89646547a@google.com>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan