Remove -latomic if unnecessary

[Matt Glazar]

My Linux toolchain supports __atomic_fetch_add_4 without -latomic.
Specifying -latomic fails because libatomic.{so,a} does not exist.

In CMake, first check if __atomic_fetch_add_4 can be used without
-latomic.  If it can't, try with -latomic.

See attached patch.

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