And support riscv64 builds with no subarch

such as https://launchpad.net/~cloudware/+livefs/ubuntu/noble/cpc/

live-build/auto/config

@@ -8,6 +8,7 @@ case $ARCH:$SUBARCH in
 	armhf:|armhf:raspi|\
 	i386:|\
 	ppc64el:|\
+	riscv64:|\
 	riscv64:icicle|riscv64:licheerv|riscv64:nezha|riscv64:unmatched|\
 	riscv64:visionfive|riscv64:visionfive2|\
 	s390x:|\