Make it possible to compile ICU both with the host and target toolset.

Make it possible to compile ICU both with the host and target toolset.

If the system ICU library is supposed to be used, still compile ICU for the
host toolset if we're using separate host and target toolsets, as the system
ICU in general is only available for the target.

BUG=none
R=mark@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=212587

[jochen (gone - plz use gerrit), 2013-07-19 12:00:48]

please review

Note that chromium does not define "want_separate_host_toolset", however, v8
defines it as 1.

To compile V8 with i18n support, we need to be able to use ICU in both the host
and the target toolset. This change will enable this.

[jochen (gone - plz use gerrit), 2013-07-19 12:08:51]

+jshin@ fyi

[Mark Mentovai, 2013-07-19 13:56:22]

LGTM

[jochen (gone - plz use gerrit), 2013-07-19 15:30:09]

Committed patchset #1 manually as r212587 (presubmit successful).