Support non-atomic targets by using portable-atomic #798

Job	Run time
loom	18s
miri	3h 26m 59s
minrust	1m 47s
cross (i686-unknown-linux-gnu)	31s
nightly	1m 15s
cross (armv7-unknown-linux-gnueabihf)	36s
no-std	51s
cross (powerpc-unknown-linux-gnu)	32s
panic=abort tests	26s
cross (powerpc64-unknown-linux-gnu)	34s
rustfmt	13s
cross (wasm32-unknown-unknown)	13s
stable (ubuntu-latest)	1m 36s
tsan	1m 20s
stable (macos-latest)	2m 14s
stable (windows-latest)	3m 4s
Publish Documentation	16s
	3h 42m 45s

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