Floss: Fix build

https://r.android.com/2058057 introduced linkage of btmanagerd with
btstack, which needs to be linked to libc++ instead of libstdc++ in
Chrome OS build.

Bug: 200732080
Tag: #floss
Test: Manual - build Floss on Chrome OS:
  USE=floss_upstream emerge-$BOARD floss

Change-Id: Ic9f0fdb70fcbd355738392cf5f1da5c267c41252