add ~riscv keyword
tatt test report: USE tests started on Mon Oct 10 09:41:37 -00 2022 FEATURES=' test' USE='' succeeded for =app-benchmarks/filebench-1.4.9.1-r1 USE='auto-completion' succeeded for =app-benchmarks/filebench-1.4.9.1-r1
riscv done all arches done