Provide AtomicWaker if portable-atomic feature is enabled, even if atomic CAS is not available #2337

Job	Run time
codegen	35s
cargo +stable build	1m 24s
cargo clippy	34s
cargo +beta build	1m 5s
cargo bench	1m 52s
cargo +nightly build	1m 17s
cargo +1.36 build (futures-{core, io, sink})	3m 42s
cargo doc	27s
cargo hack check --feature-powerset	4m 55s
cargo fmt	9s
cargo minimal-versions build	38s
cargo miri test	13m 50s
cargo build --target thumbv7m-none-eabi	40s
cargo test -Z sanitizer=address	5m 43s
cargo test -Z sanitizer=memory	4m 10s
cargo build --target thumbv6m-none-eabi	43s
cargo test (ubuntu-latest)	4m 2s
cargo test -Z sanitizer=thread	3m 26s
cargo test (macos-latest)	6m 28s
cargo +1.56 build	2m 26s
cargo test (windows-latest)	7m 6s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	5m 35s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	12m 3s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	12m 40s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	5m 9s
	1h 40m 39s

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