Remove powerpc left-overs

Remove powerpc left-overs

There since the powerpc -> macppc move/rename.

KERN_AS   usage disappeared in
	commit 5b7db11d478192c5908038bb1345e7d51cc35c8e
	Author: rahnds <rahnds@openbsd.org>
	Date:   Mon May 5 16:47:15 1997 +0000

	only build one version of the libraries.

REAL_VIRT usage disappeared in
	commit 08e027d6bb9ca863bfc44a1aa6957ff3a242e2f4
	Author: rahnds <rahnds@openbsd.org>
	Date:   Thu Apr 27 12:36:29 2000 +0000

	Fixes to xcoff bootloader to allow it to execute and load kernels for OpenBSD.

Make agrees that these are defined but not used:

	$ make -p            | grep -e KERN_AS -e REAL_VIRT
	KERN_AS          = library
	REAL_VIRT        = -v
	$ make -C ofwboot -p | grep -e KERN_AS -e REAL_VIRT
	KERN_AS          = library
	REAL_VIRT        = -v

No object change.
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