target/riscv: Fix mcycle/minstret increment behavior

target/riscv: Fix mcycle/minstret increment behavior

The mcycle/minstret counter's stop flag is mistakenly updated on a copy
on stack. Thus the counter increments even when the CY/IR bit in the
mcountinhibit register is set. This commit corrects its behavior.

Fixes: 3780e33732f88 (target/riscv: Support mcycle/minstret write operation)
Signed-off-by: Xu Lu <luxu.kernel@bytedance.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>