impl atomic ops for fallback backend #3307

Job	Run time
macOS / release / homebrew-clang	4m 55s
macOS / debug / homebrew-clang	4m 11s
macOS / release / system-clang	3m 39s
macOS / debug / system-clang	3m 41s
Windows / release / msvc	25m 25s
Windows / release / clang-cl	26m 21s
Windows / release / llvm	21m 47s
Windows / debug / msvc	26m 43s
Windows / debug / clang-cl	24m 44s
Windows / debug / llvm	21m 6s
	2h 42m 32s

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