gcc-cross: pass -Werror=poison-system-directories to compiler stages

gcc-cross: pass -Werror=poison-system-directories to compiler stages

The -Werror=poison-system-directories argument was being passed to runs
where _just_ CPP was being invoked, so it wasn't passed to runs which
also compile.  Add the options to cc1_options so that compile runs also
have fatal poisoning errors.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>