runtime: review needed before freeze

[Jorropo]

Hello I've got this small patch which is relatively important to me: https://go-review.googlesource.com/c/go/+/751342 

It impacts 32 bits linux platforms running linux < 5.1.

In theory it adds dead code, since it adds safety checks that currently are impossible to trigger.

The point is that if I've got things wrong, the runtime would crash rather than doing syscalls with truncated corrupted time values.

As you can guess without this it is pretty much impossible for me to fix any of such corruption bugs in 1.27.x if we silently perform syscalls with corrupted arguments.

Let me know if there is anything that needs to happen to get this in before the freeze,
thanks.