Add support for QNX Neutrino part2

pqnx.h was not added in the previous CL by mistake
(https://codereview.chromium.org/195973005/ ).

A spurious tab in icu.gyp was removed.

README.chromium was updated to clarify how pqnx.h was generated.
platform.qnx.patch was made against a copy of plinux.h

BUG=NONE
TEST=icu is built properly on qnx. After DEPS roll, net_unittests:*IDN* and base_unittests:*String* all pass on qnx.`
R=mark@chromium.org

Review URL: https://codereview.chromium.org/203823003

git-svn-id: https://src.chromium.org/chrome/trunk/deps/third_party/icu46@258166 4ff67af0-8c30-449e-8e8b-ad334ec8d88c