Define _LIBCPP_ABI_UNSTABLE manually when building libcxxabi

This is set via a cmake option when building libcxx, but libcxxabi
uses the raw libcxx headers from its include directory before they
are installed.

This fixes cases of inconsistent classes seen while compiling libcxxabi
and libcxx. With potential future patches to clang to adjust how
classes are exported with dllexport, this change is needed to avoid
conflicts.

build-libcxx.sh

@@ -167,7 +167,7 @@ build_all() {
             -DLIBCXXABI_ENABLE_NEW_DELETE_DEFINITIONS=OFF \
             -DLLVM_NO_OLD_LIBSTDCXX=TRUE \
             -DCXX_SUPPORTS_CXX11=TRUE \
-            -DCMAKE_CXX_FLAGS="$LIBCXXABI_VISIBILITY_FLAGS -D_LIBCPP_HAS_THREAD_API_WIN32 -w" \
+            -DCMAKE_CXX_FLAGS="$LIBCXXABI_VISIBILITY_FLAGS -D_LIBCPP_HAS_THREAD_API_WIN32 -D_LIBCPP_ABI_UNSTABLE -w" \
             ..
         make -j$CORES
         cd ..