rs6000: Enable overlapped by-pieces operations

This patch enables overlapped by-piece operations by defining
TARGET_OVERLAP_OP_BY_PIECES_P to true.  On rs6000, default move/set/clear
ratio is 2.  So the overlap is only enabled with compare by-pieces.

gcc/
	* config/rs6000/rs6000.cc (TARGET_OVERLAP_OP_BY_PIECES_P): Define.

gcc/testsuite/
	* gcc.target/powerpc/block-cmp-9.c: New.