testsuite, rs6000: Fix some run cases with appropriate _hw

When cleaning up the remaining powerpc_{vsx,altivec}_ok test
cases, I found some dg-do run test cases which should check
for the appropriate {p8vector,vmx}_hw check instead.  This
patch is to adjust them accordingly.

gcc/testsuite/ChangeLog:

	* gcc.target/powerpc/swaps-p8-46.c: Check for p8vector_hw rather than
	powerpc_vsx_ok.
	* gcc.target/powerpc/ppc64-abi-2.c: Check for vmx_hw rather than
	powerpc_altivec_ok.
	* gcc.target/powerpc/pr96139-c.c: Likewise.