time: Lack of monotonic time support on Solaris

[AudriusButkevicius]

What version of Go are you using (go version)?

1.12.17

Does this issue reproduce with the latest release?

I have no means of testing due to lack of access to the platform, but looking at the code it seems it should still happen on master.

What operating system and processor architecture are you using (go env)?

solaris amd64

What did you do?

Compared two values of time.Now() not that far from each other.
The latter time.Now() value was smaller than the value acquired first.

Essentially:

	x := time.Now()
	y := time.Now()
	fmt.Println(y.Sub(x))

resulted in a negative difference

What did you expect to see?

Difference in time to be zero or positive

What did you see instead?

The time difference was negative 87ms.

It seems that the problem stems from:
https://github.com/golang/go/blob/master/src/runtime/sys_solaris_amd64.s#L42

which still uses CLOCK_REALTIME

This originates from: quic-go/quic-go#2059

[ianlancetaylor]

It does seem like we should change walltime to use CLOCK_REALTIME and change nanotime to use CLOCK_MONOTONIC. That's how it works on GNU/Linux.

[gopherbot]

Change https://golang.org/cl/199502 mentions this issue: runtime: make nanotime use monotonic clock in Solaris