Use cfg(target_has_atomic) on no-std targets #2419

Job	Run time
cargo +1.56 build	1m 2s
cargo test (ubuntu-latest)	3m 15s
cargo test (macos-latest)	4m 59s
cargo bench	54s
cargo +1.36 build (futures-{core, io, sink})	36s
cargo test (windows-latest)	5m 17s
cargo +stable build	42s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	3m 36s
cargo +beta build	46s
cargo hack check --feature-powerset	2m 28s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	6m 9s
cargo minimal-versions build	27s
cargo +nightly build	47s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	6m 41s
cargo miri test	8m 34s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	3m 32s
cargo build --target thumbv7m-none-eabi	29s
cargo test -Z sanitizer=address	5m 32s
cargo build --target thumbv6m-none-eabi	32s
cargo test -Z sanitizer=memory	4m 5s
cargo test -Z sanitizer=thread	2m 49s
cargo clippy	27s
cargo fmt	11s
	1h 3m 50s

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