target/riscv: Trigger interrupt on MIP update asynchronously

target/riscv: Trigger interrupt on MIP update asynchronously

The requirement of holding the iothread_mutex is burdersome when
swapping the background and foreground registers in the Hypervisor
extension. To avoid the requrirement let's set the interrupt
asynchronously.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Palmer Dabbelt <palmer@sifive.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>