Add U_PF_NATIVE_CLIENT

NaCl has to be identified by '__native_client__'. Currently, we assume that
NaCl has a signature of Linux (e.g. __linux___), but that is not the case.

Add a new platform (U_PF_NATIVE_CLIENT) to platform.h and treat it as
Linux-based.

Upstream bug : http://bugs.icu-project.org/trac/ticket/11033

BUG=NONE
TEST=NaCl builds fine when ICU is rolled to this revision.
R=ncbray@chromium.org

Review URL: https://codereview.chromium.org/419013002

git-svn-id: http://src.chromium.org/svn/trunk/deps/third_party/icu52@285387 4ff67af0-8c30-449e-8e8b-ad334ec8d88c
2 files changed