[libc++abi] Make sure we link in CrashReporterClient.a when it's present

When building the system libc++abi for Apple, we use CrashReporterClient
to provide better crash logs when calling abort(). This is exemplified by
the fact that we test for the presence of <CrashReporterClient.h> in
abort_message.cpp.

However, we must link against CrashReporterClient.a in order to get that
functionality, otherwise we get a linking error.