[gn] Support for building libcxxabi

This change introduces support for building libcxxabi. The library
build should be complete, but not all CMake options have been
replicated in GN. We also don't support tests yet.

We only support two stage build at the moment.

Differential Revision: https://reviews.llvm.org/D60372

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@359805 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed