target/riscv: Backup/restore mstatus.SD bit when virtual register swapped

target/riscv: Backup/restore mstatus.SD bit when virtual register swapped

When virtual registers are swapped, mstatus.SD bit should also be
backed up/restored. Otherwise, mstatus.SD bit will be incorrectly kept
across the world switches.

Signed-off-by: Frank Chang <frank.chang@sifive.com>
Reviewed-by: Vincent Chen <vincent.chen@sifive.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20210914013717.881430-1-frank.chang@sifive.com
[ Changes by AF:
 - Convert variable to a uint64_t to fix clang error
]
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>