runtime: remove atomic store requirement on pageAlloc.chunks
runtime: remove atomic store requirement on pageAlloc.chunks

pageAlloc.chunks used to require an atomic store when growing the heap
because the scavenger would look at the list without locking the heap
lock. However, the scavenger doesn't do that anymore, and it looks like
nothing really does at all.

This change updates the comment and makes the store non-atomic.

Change-Id: Ib452d147861060f9f6e74e2d98ee111cf89ce8f5
Reviewed-on: https://go-review.googlesource.com/c/go/+/429219
Auto-Submit: Michael Knyszek <4473450dc5c88f06e1fb787f303d7805ec2aee65@google.com>
Reviewed-by: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Run-TryBot: Michael Knyszek <4473450dc5c88f06e1fb787f303d7805ec2aee65@google.com>
Reviewed-by: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go