wip: atomics in fallback backend #3305

Job	Run time
macOS / release / homebrew-clang	5m 13s
Windows / release / msvc	30m 16s
macOS / debug / homebrew-clang	5m 13s
Windows / release / clang-cl	23m 42s
macOS / release / system-clang	4m 14s
Windows / release / llvm	26m 29s
macOS / debug / system-clang	3m 55s
Windows / debug / msvc	21m 37s
Windows / debug / clang-cl	25m 14s
Windows / debug / llvm	15m 19s
	2h 41m 12s

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