Fix abort_message.cpp for the NDK.
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: 6a9f6578bdee8d959f0ed58970538b4ab6004734@226310 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libcxxabi,llvm-mirror/libcxxabi,llvm-mirror/libcxxabi,llvm-mirror/libcxxabi,llvm-mirror/libcxxabi