add ~riscv keyword
Looks good on riscv USE tests started on Tue Nov 1 16:42:01 -00 2022 FEATURES=' test' USE='' succeeded for =net-proxy/ziproxy-3.3.1-r2 USE='-sasl xinetd' succeeded for =net-proxy/ziproxy-3.3.1-r2
riscv done all arches done