exec.c: Record watchpoint fault address and direction

When we check whether we've hit a watchpoint we know the address
that we were attempting to access and whether it was a read or a
write. Record this information in the CPUWatchpoint struct so that
target-specific code can report it to the guest.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>