rs6000/testsuite: Don't use powerpc64 effective target

rs6000/testsuite: Don't use powerpc64 effective target

The powerpc64 effective target unfortunately does not mean the target
has 64-bit instructions enabled (i.e., -mpowerpc64): instead, it means
that the assembler supports it.

Let's use the lp64 effective target instead for these tests.

2020-05-15  Segher Boessenkool  <segher@kernel.crashing.org>

gcc/testsuite/
	* gcc.target/powerpc/cntlzdm-0.c: Use lp64 instead of powerpc64.
	* gcc.target/powerpc/cntlzdm-1.c: Ditto.
	* gcc.target/powerpc/cnttzdm-1.c: Ditto.
	* gcc.target/powerpc/pdep-0.c: Ditto.
	* gcc.target/powerpc/pdep-1.c: Ditto.
	* gcc.target/powerpc/pextd-0.c: Ditto.
	* gcc.target/powerpc/pextd-1.c: Ditto.