rs6000: Improve .machine

rs6000: Improve .machine

This adds more correct .machine for most older CPUs.  It should be
conservative in the sense that everything we handled before we handle at
least as well now.  This does not yet revamp the server CPU handling, it
is too risky at this point in time.

Tested on powerpc64-linux {-m32,-m64}.  Also manually tested with all
-mcpu=, and the output of that passed through the GNU assembler.

2022-03-04  Segher Boessenkool  <segher@kernel.crashing.org>

	* config/rs6000/rs6000.cc (rs6000_machine_from_flags): Restructure a
	bit.  Handle most older CPUs.