Port amd64 (and i386) XEN builds still failing due to acpi

[Paul Goyette <paul%whooppee.com@localhost>]

#      link  XEN3_DOM0/netbsd
/build/netbsd-local/tools/x86_64/amd64/bin/x86_64--netbsd-ld -Map netbsd.map 
--cref -T /build/netbsd-local/src/sys/arch/amd64/conf/kern.ldscript.Xen -Ttext 
0xffffffff80100000 -e start -X -o netbsd ${SYSTEM_OBJ} ${EXTRA_OBJ} vers.o
acpi.o: In function `acpi_attach':
/build/netbsd-local/src/sys/dev/acpi/acpi.c:447: undefined reference to 
`acpi_md_pdc'

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