Expose libcompiler_rt-extras to the NDK.

The NDK can't fully switch to compiler-rt until the unwinders have
been sorted out, so it needs this for the time being.

Test: build/soong/scripts/build-ndk-prebuilts.sh
Bug: https://github.com/android-ndk/ndk/issues/294
Change-Id: I65516f4369f4e6c39b54e3c049df224b8d908d31