Clint: Implement write, fix bug int32->int64
Clint: Implement write, fix bug int32->int64

Put the rtc in an MVar to be able to update it from an independent thread.

mit-plv/riscv-semantics,mit-plv/riscv-semantics,mit-plv/riscv-semantics,mit-plv/riscv-semantics,mit-plv/riscv-semantics