Fix abort_message.cpp for the NDK.

The NDK doesn't have access to `android/set_abort_message.h`, so use
an extern declaration instead for API 21. For older releases, just use
`__assert2`, which will report to logcat and/or the tombstone for some
older releases.


git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@226310 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed