RISC-V: Allow S-mode mxr access when priv ISA >= v1.10

RISC-V: Allow S-mode mxr access when priv ISA >= v1.10

The mstatus.MXR alias in sstatus should only be writable
by S-mode if the privileged ISA version >= v1.10. Also MXR
was masked in sstatus CSR read but not sstatus CSR writes.
Now we correctly mask sstatus.mxr in both read and write.

Cc: Sagar Karandikar <sagark@eecs.berkeley.edu>
Cc: Bastian Koppelmann <kbastian@mail.uni-paderborn.de>
Cc: Palmer Dabbelt <palmer@sifive.com>
Cc: Alistair Francis <Alistair.Francis@wdc.com>
Signed-off-by: Michael Clark <mjc@sifive.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>