Remove libstdc++ dependency from host libbacktrace

The libc++ version of libbacktrace on the host was still linking against
libstdc++, and this was causing problems in art.

(cherry picked from commit a3a8f6d2ab16afaa3018406c694c7d63929f2a83)

Change-Id: Id511dd9cca05c3aa0de2ad13682675ea4ac6424d
diff --git a/libbacktrace/Android.build.mk b/libbacktrace/Android.build.mk
index dc73f9f..9882e31 100644
--- a/libbacktrace/Android.build.mk
+++ b/libbacktrace/Android.build.mk
@@ -72,6 +72,9 @@
 ifeq ($(build_type),host)
   # Only build if host builds are supported.
   ifeq ($(build_host),true)
+    ifneq ($($(module)_libc++),)
+      include external/libcxx/libcxx.mk
+    endif
     include $(BUILD_HOST_$(build_target))
   endif
 endif