sim: sync build_warnings handling with gdb

sim: sync build_warnings handling with gdb

The sim code gets the logic for SIM_AC_OPTION_WARNINGS from gdb, but
it hasn't been updated in a good long while.  Sync with the latest
gdb code.

There is a sim specific change in here: we disable -Werror for now.
This is because all sim code atm contains warnings.  Will probably
have to slowly add a white list of targets which can tolerate this
until everyone is updated.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>