runtime/internal/atomic: add Bool
runtime/internal/atomic: add Bool

This change adds the Bool type, a convenient wrapper around Uint8 for
atomic bool values.

Change-Id: I86127d6f213b730d6999db5718ca1a5af0c5b538
Reviewed-on: https://go-review.googlesource.com/c/go/+/393395
Reviewed-by: Michael Pratt <e7f1edc6d5aa9f6c8c377425db8f1a5816cbbb88@google.com>
Reviewed-by: Austin Clements <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>
Trust: Michael Knyszek <4473450dc5c88f06e1fb787f303d7805ec2aee65@google.com>

golang/go,CAFxX/go,nawawi/go,CAFxX/go,CAFxX/go,nawawi/go,golang/go,nawawi/go,nawawi/go,nawawi/go,CAFxX/go,CAFxX/go,golang/go,nawawi/go,CAFxX/go,golang/go,CAFxX/go,golang/go,golang/go,golang/go,golang/go,CAFxX/go,nawawi/go,nawawi/go