Remove '-Wextra' as default user flags for the Linux clang targets

We have '--strict-warnings' for this kind of stuff...  also, user
flags are added last, so this overrides any warning supression
--strict-warnings may put in place (for good reasons).

Fixes #5609

Reviewed-by: Tim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5612)