If you:

TCLIBC=musl bitbake unwind
TCLIBC=musl bitbake gcc-runtime -c cleansstate
TCLIBC=musl bitbake gcc-runtime

you will see libstdc++ fail to build due to finding libunwind's header file.

Khem: "When we build any of gcc components they expect to use internal version 
and that works with glibc based gcc since the search headers first look into gcc 
headers, however with musl the gcc headers are searched after the standard 
headers ( which is by design the right thing )."

This patch hacks around the issue by looking for a define used during gcc-runtime's 
build and skipping to the internal header in that case.

[YOCTO #10129]

RP 2016/8/18

Upstream-Status: Inappropriate [really need to fix gcc]

Index: git/include/unwind.h
===================================================================
--- git.orig/include/unwind.h
+++ git/include/unwind.h
@@ -23,6 +23,10 @@ LIABLE FOR ANY CLAIM, DAMAGES OR OTHER L
 OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
 WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.  */
 
+#ifdef _GLIBCXX_SHARED
+#include_next <unwind.h>
+#endif
+
 #ifndef _UNWIND_H
 #define _UNWIND_H