build: fix --with-intl=system-icu for x-compile

* add toolset stuff to icu-system.gyp
* update docs

Fixes: https://github.com/nodejs/node/issues/3801
PR-URL: https://github.com/nodejs/node/pull/3808


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>