Fix StaticSingletonManager inline fast path under gcc
Fix StaticSingletonManager inline fast path under gcc

Summary: [Folly] Fix StaticSingletonManager inline fast path under gcc - code size for default-initialized `static std::atomic<...>` local is large, but when value-initialized is trivial.

Reviewed By: andriigrynenko

Differential Revision: D14018861

fbshipit-source-id: e767ea3e46c3e3317f7b60008bc77f64edb2dea0

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