The null pointer check is never needed for correct code, only to
gracefully handle undefined cases. Add __builtin_expect to be sure that
we don't pessimize the valid uses.

libstdc++-v3/ChangeLog:

	* libsupc++/dyncast.cc (__dynamic_cast): Add __builtin_expect to
	precondition check.

Tested powerpc64le-linux. Committed to trunk.