Use new.cpp from libcxx instead of gabi++

The header is stubbed out in gabi++ already, but both new implementations
are compiled. Let's stub out the gabi++ one.

Change-Id: I5145dd6c630205ceddc3c484ae18af753ac72165
Merge pull request #4 from awong-dev/uselibcxxnewcpp

Use new.cpp from libcxx instead of gabi++