Tweak throw_exception condition
Tweak throw_exception condition

Summary: [Folly] Tweak throw_exception condition, fixing reference to `__GNUC__` and restructuring to make adding checks for alternative compilers more trivial.

Reviewed By: mzlee

Differential Revision: D7102307

fbshipit-source-id: e4e00ded71f37db8fa93822744b608b42960a58a

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