Allow packetdrill to be built on powerpc and powerpc64

gtests/net/packetdrill/Makefile.FreeBSD

+ARCH != uname -p
 packetdrill-ext-libs := -lprocstat -lutil -lkvm -lelf -lpthread -lpcap
-CFLAGS = -Wall -Werror -Wno-unknown-warning-option -Wno-address-of-packed-member
+CFLAGS = -Wall -Werror
+.if ${ARCH} != "powerpc" && ${ARCH} != "powerpc64"
+  += -Wno-unknown-warning-option -Wno-address-of-packed-member
+.endif
 .include "Makefile.common"