target/riscv: Save counter values during countinhibit update

target/riscv: Save counter values during countinhibit update

Currently, if a counter monitoring cycle/instret is stopped via
mcountinhibit we just update the state while the value is saved
during the next read. This is not accurate as the read may happen
many cycles after the counter is stopped. Ideally, the read should
return the value saved when the counter is stopped.

Thus, save the value of the counter during the inhibit update
operation and return that value during the read if corresponding bit
in mcountihibit is set.

Acked-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Signed-off-by: Atish Patra <atishp@rivosinc.com>
Message-ID: <20240711-smcntrpmf_v7-v8-8-b7c38ae7b263@rivosinc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>