This makes the Linux GN builds use the Chromium copy of binutils and
also makes all platforms use the "custom libcxx" which is libc++.
We do this so that the build doesn't depend on anything from the system
on Linux because our bots are still on trusty with very old libstdc++
and linkers.
Change-Id: I0570fb474f2945c3565e78a56aba66c1a2619afc