runtime: convert schedt.lastpoll to atomic type

runtime: convert schedt.lastpoll to atomic type

Note that this changes the type from uint64 to int64, the type used by
nanotime(). It also adds an atomic load in pollWork(), which used to use
a non-atomic load.

For #53821.

Change-Id: I6173c90f20bfdc0e0a4bc3a7b1c798d1c429fff5
Reviewed-on: https://go-review.googlesource.com/c/go/+/419442
Run-TryBot: Michael Pratt <mpratt@google.com>
Reviewed-by: Austin Clements <austin@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>