Add atomic fallback for CXX without std::atomic #186

Job	Run time
Check codebase format with clang-format	5s
Test multicast and unicast fragmentation	9s
Modular build on ubuntu-latest (1, 1, 1, 1)	10s
Modular build on ubuntu-latest (1, 1, 1, 0)	9s
Build and test raweth transport on ubuntu-latest (1)	8s
Modular build on ubuntu-latest (1, 1, 0, 1)	23s
Run unit tests on ubuntu-latest	2m 8s
Build and test raweth transport on ubuntu-latest (0)	20s
Modular build on ubuntu-latest (1, 1, 0, 0)	7s
Modular build on ubuntu-latest (1, 0, 1, 1)	9s
Build and test in single thread on ubuntu-latest	13s
Modular build on ubuntu-latest (1, 0, 1, 0)	12s
Modular build on ubuntu-latest (1, 0, 0, 1)	11s
Modular build on ubuntu-latest (1, 0, 0, 0)	24s
Modular build on ubuntu-latest (0, 1, 1, 1)	12s
Modular build on ubuntu-latest (0, 1, 1, 0)	22s
Modular build on ubuntu-latest (0, 1, 0, 1)	20s
Modular build on ubuntu-latest (0, 1, 0, 0)	25s
Modular build on ubuntu-latest (0, 0, 1, 1)	9s
Modular build on ubuntu-latest (0, 0, 1, 0)	9s
Modular build on ubuntu-latest (0, 0, 0, 1)	21s
Modular build on ubuntu-latest (0, 0, 0, 0)	26s
	7m 12s

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