[gn] Support for building libunwind

This change introduces support for building libuwind. 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/D60370

git-svn-id: https://llvm.org/svn/llvm-project/libunwind/trunk@359804 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed