Merge pull request #2161 from isuruf/c11atomics #2965

Job	Run time
GCC build regression check (no tests)	1m 54s
macOS-M1, Clang (x3)	9m 15s
Alpine Linux, musl, 32-bit (assert, x1.5)	14m 15s
MSVC (x1)	10m 23s
MinGW GCC (x0.5)	15m 39s
Nemo.jl	21m 25s
Ubuntu Clang, BLAS and examples (x5)	12m 51s
Ubuntu GCC, Code Coverage (x10)	30m 10s
Ubuntu GCC with NTL (assert, x2)	8m 1s
	2h 3m 53s

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