virtual/mpi still provibe sys-cluster/mpich, we should support all available mpi providers on riscv. add ~riscv keyword.
USE tests started on Wed Dec 7 08:37:44 PM CST 2022 FEATURES=' test' USE='' succeeded for =sys-cluster/mpich-3.4.3 USE='cxx doc fortran -mpi-threads -romio -threads' succeeded for =sys-cluster/mpich-3.4.3 USE='cxx -doc fortran mpi-threads -romio -threads' : REQUIRED_USE not satisfied (probably) for =sys-cluster/mpich-3.4.3 USE='-cxx -doc -fortran -mpi-threads -romio threads' succeeded for =sys-cluster/mpich-3.4.3 USE='-cxx -doc fortran -mpi-threads -romio threads' succeeded for =sys-cluster/mpich-3.4.3 USE='cxx -doc fortran -mpi-threads -romio threads' succeeded for =sys-cluster/mpich-3.4.3 USE='-cxx -doc fortran mpi-threads -romio threads' succeeded for =sys-cluster/mpich-3.4.3 USE='-cxx doc -fortran -mpi-threads romio threads' succeeded for =sys-cluster/mpich-3.4.3 USE='-cxx -doc fortran -mpi-threads romio threads' succeeded for =sys-cluster/mpich-3.4.3 USE='-cxx doc fortran -mpi-threads romio threads' succeeded for =sys-cluster/mpich-3.4.3 USE='-cxx doc -fortran mpi-threads romio threads' succeeded for =sys-cluster/mpich-3.4.3 USE='-cxx -doc fortran mpi-threads romio threads' succeeded for =sys-cluster/mpich-3.4.3 USE='-cxx doc fortran mpi-threads romio threads' succeeded for =sys-cluster/mpich-3.4.3
Cause REQUIRED_USE="mpi-threads? ( threads )", this warning should be ignored, so every USE tests succeeded. USE='cxx -doc fortran mpi-threads -romio -threads' : REQUIRED_USE not satisfied (probably) for =sys-cluster/mpich-3.4.3
riscv done all arches done