stream_base: `.writev()` has limited support

Only TCP and JSStream do support `.writev()` on all platforms at the
moment. Ensure that it won't be enabled everywhere.

Fix: https://github.com/iojs/io.js/issues/995
PR-URL: https://github.com/iojs/io.js/pull/1008


Reviewed-by: Bert Belder <bertbelder@gmail.com>
Reviewed-By: Rod Vagg <rod@vagg.org>