Update portable-atomic to 1.0 (#2688)

Author: taiki-e

futures-core/Cargo.toml

@@ -21,7 +21,7 @@ unstable = []
 cfg-target-has-atomic = []
 
 [dependencies]
-portable-atomic = { version = "0.3.15", default-features = false, optional = true }
+portable-atomic = { version = "1", default-features = false, optional = true }
 
 [dev-dependencies]
 futures = { path = "../futures" }