hal-x86_64: want API for local APIC timer in oneshot mode

[hawkw]

currently, the x86_64 HAL has a nice method for configuring the local APIC timer as a periodic timer, but there's no corresponding API for using it as a one-shot timer.

we use the oneshot timer internally for calibrating the local APIC timer frequency, here:

mycelium/hal-x86_64/src/interrupt/apic/local.rs

Lines 164 to 188 in b0d028f

	unsafe {
	// set timer divisor to 16
	self.write_register(TIMER_DIVISOR, 0b11);
	// set initial count to -1
	self.write_register(TIMER_INITIAL_COUNT, -1i32 as u32);
	// start the timer
	self.write_register(
	LVT_TIMER,
	LvtTimer::new().with(LvtTimer::MODE, TimerMode::OneShot),
	);
	}
	// use the PIT to sleep for 10ms
	pit.sleep_blocking(Duration::from_millis(10))
	.expect("the PIT should be able to send a 10ms interrupt...");
	unsafe {
	// stop the timer
	self.write_register(
	LVT_TIMER,
	LvtTimer::new().with(LvtTimer::MODE, TimerMode::Periodic),
	);
	}

but we don't provide an interface for it. we should probably fix that.