RISC-V: recognize S/U mode bits in print_isa

Removes the warning about an unsupported ISA when reading /proc/cpuinfo
on QEMU. The "S" extension is not being returned as it is not accessible
from userspace.

Signed-off-by: Patrick Stählin <me@packi.ch>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>