Also add support for riscv64 builds with 'generic' subarch

CPC team builds use null subarch but ubuntu-cdimage builds use 'generic'
subarch

https://launchpad.net/~ubuntu-cdimage/+livefs/ubuntu/noble/cpc/+build/542136

live-build/auto/config

@@ -8,9 +8,8 @@ case $ARCH:$SUBARCH in
 	armhf:|armhf:raspi|\
 	i386:|\
 	ppc64el:|\
-	riscv64:|\
-	riscv64:icicle|riscv64:licheerv|riscv64:nezha|riscv64:unmatched|\
-	riscv64:visionfive|riscv64:visionfive2|\
+	riscv64:|riscv64:generic|riscv64:icicle|riscv64:licheerv|\
+	riscv64:nezha|riscv64:unmatched|riscv64:visionfive|riscv64:visionfive2|\
 	s390x:|\
 	*appliance*)
 		;;