rs6000: Do not use rs6000_cpu for .machine ppc and ppc64 (PR104829)

rs6000: Do not use rs6000_cpu for .machine ppc and ppc64 (PR104829)

Fixes: 77eccbf39ed5

rs6000.h has
  #define PROCESSOR_POWERPC   PROCESSOR_PPC604
  #define PROCESSOR_POWERPC64 PROCESSOR_RS64A
which means that if you use things like  -mcpu=powerpc -mvsx  it will no
longer work after my latest .machine patch.  This causes GCC build errors
in some cases, not a good idea (even if the errors are actually
pre-existing: using -mvsx with a machine that does not have VSX cannot
work properly).

2022-03-11  Segher Boessenkool  <segher@kernel.crashing.org>

	PR target/104829
	* config/rs6000/rs6000.cc (rs6000_machine_from_flags): Don't output
	"ppc" and "ppc64" based on rs6000_cpu.