Exclude libc++abi target when use_custom_libcxx=false

//base pulls in //buildtools/third_party/libc++abi:cxa_demangle_stub.  Doing
this causes gn to see the file buildtools/third_party/libc++abi/BUILD.gn, which
means all targets from it will get added to gn_all.

We want to avoid this in cases where use_custom_libcxx=false.  This was causing
the build to fail on Android because libc++(abi) was pulling in headers from
Android's copy of libc++.

BUG=767901
R=thakis

Change-Id: I3578bf4d7917c5385769956d11fadc1200cd16fd
1 file changed