build: i18n: fix icu toolset dependencies

So what I did here is to make the icutools library actually contain the
entire ICU dependencies needed for host-side tools. Sadly, this will
build ICU twice, but avoids conflicts between host and target side.
This all seems like a gyp bug of some sort, but without docs for
toolsets, who’s to say?

I removed the icuio library as a separate target, because it was only
used by the host-side tools.

PR-URL: https://github.com/joyent/node/pull/8681
Reviewed-by: Trevor Norris <trev.norris@gmail.com>