Support CLOCK_BOOTTIME

[prattmic]

We could likely make CLOCK_BOOTTIME equivalent to CLOCK_MONOTONIC, as CLOCK_MONOTONIC already includes time spent not running during save / restore, which is the closest thing we have to "suspend".

gvisor/pkg/sentry/kernel/timekeeper.go

Lines 119 to 129 in 8f46349

	// Compute the offset of the monotonic clock from the base Clocks.
	//
	// In a fresh (not restored) sentry, monotonic time starts at zero.
	//
	// In a restored sentry, monotonic time jumps forward by approximately
	// the same amount as real time. There are no guarantees here, we are
	// just making a best-effort attempt to to make it appear that the app
	// was simply not scheduled for a long period, rather than that the
	// real time clock was changed.
	//
	// If real time went backwards, it remains the same.