rs6000: Update the processor defaults for FreeBSD

gcc/ChangeLog:

2020-12-13  Piotr Kubaj  <pkubaj@FreeBSD.org>
	    Gerald Pfeifer  <gerald@pfeifer.com>

	* config/rs6000/freebsd64.h (PROCESSOR_DEFAULT): Update
	to PROCESSOR_PPC7450.
	(PROCESSOR_DEFAULT64): Update to PROCESSOR_POWER8.