Revert "sync: convert Once.done to atomic type"

Revert "sync: convert Once.done to atomic type"

This reverts commit CL 427140.

Reason for revert: Comments say that done should be the first field.

Change-Id: Id131da064146b44e1182289546aeb877867e63cc
Reviewed-on: https://go-review.googlesource.com/c/go/+/428638
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