[PNaCl Toolchain] Use static libc++ from Chrome toolchain on Mac
[PNaCl Toolchain] Use static libc++ from Chrome toolchain on Mac

https://codereview.chromium.org/1572753002/ stopped building our custom libc++
but failed to add the linker path to the static libc++ library, resulting in
a dependence on the system libc++ dylib. This fails for OSX 10.6 systems, which
don't have the dylib.

BUG=None
R=mcgrathr@chromium.org

Review URL: https://codereview.chromium.org/1586243003

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