testsuite, rs6000: Adjust pr78056-[1357].c and remove pr78056-[246].c

When cleaning up the remaining powerpc_{vsx,altivec}_ok test
cases, I found some issues are related to pr78056-*.c.
Firstly, the test points of pr78056-[246].c are no longer
available since r9-3164 drops many HAVE_AS_* and the expected
warning are dropped together, so this patch is to remove them.
Secondly, pr78056-1.c and pr78056-3.c include altivec.h but
don't use any builtins, checking powerpc_altivec is enough
(don't need to check powerpc_vsx).  And pr78056-5.c doesn't
require any altivec/vsx feature, so powerpc_vsx_ok can be
removed.  Lastly, pr78056-7.c should just use powerpc_fprs
instead of dfp_hw as it only cares about insn fcpsgn.

gcc/testsuite/ChangeLog:

	* gcc.target/powerpc/pr78056-1.c: Check for powerpc_altivec rather than
	powerpc_vsx.
	* gcc.target/powerpc/pr78056-3.c: Likewise.
	* gcc.target/powerpc/pr78056-5.c: Drop powerpc_vsx_ok check.
	* gcc.target/powerpc/pr78056-7.c: Check for powerpc_fprs rather than
	dfp_hw.
	* gcc.target/powerpc/pr78056-2.c: Remove.
	* gcc.target/powerpc/pr78056-4.c: Remove.
	* gcc.target/powerpc/pr78056-6.c: Remove.