Let build/tools/build-cxx-stl.sh now use ninja instead of make.

Makes building libcxx about 2s faster, and has a nicer UI.

Change-Id: I12d227b890dbe39f91e4d792537336705c0003fd
Merge pull request #1 from awong-dev/ninja

Let build/tools/build-cxx-stl.sh now use ninja instead of make.