Work around incorrect use of libc++_shared.so

This patch removes libc++ from the dependencies for the runtime
libraries, and instead adds libcxxabi.a and libatomic.a. We also now
have to tell clang to not automatically link libstdc++.so, which it was
incorrectly doing as well.

Bug: http://b/28196329
Test: toolchain/llvm/android/build.py and boot a device with it :)
Change-Id: I7a64215c4d9b1d43c78f672cf44e8b39b19b0bfc