PNaCl toolchain: Add 'nostdinc++' to build of host tools against libc++
PNaCl toolchain: Add 'nostdinc++' to build of host tools against libc++

Previously, the '-stdlib=libc++' flag was sufficient to remove the system C++
headers from the search path because there were no libc++ headers available.
With the recent Chromium clang update, the compiler includes its own libc++
headers, which conflict with our libc++ headers that we manually include.
So also add -nostdinc++ so that we only get one set of libc++ headers.

R=mcgrathr@chromium.org, stichnot@chromium.org

Review URL: https://codereview.chromium.org/1604353002 .

sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client