FEAT_WFxT introduces new instructions WFIT and WFET, which are like
the existing WFI and WFE but allow the guest to pass a timeout value
in a register.  The instructions will wait for an interrupt/event as
usual, but will also stop waiting when the value of CNTVCT_EL0 is
greater than or equal to the specified timeout value.

We implement WFIT by setting up a timer to expire at the right
point; when the timer expires it sets the EXITTB interrupt, which
will cause the CPU to leave the halted state. If we come out of
halt for some other reason, we unset the pending timer.

We implement WFET as a nop, which is architecturally permitted and
matches the way we currently make WFE a nop.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20240430140035.3889879-3-peter.maydell@linaro.org