disable stack madvise optimization on non-Linux platforms
disable stack madvise optimization on non-Linux platforms

Summary:
This diff strengthens the preconditions for attempting to use
optimize by madvising away idle stacks, because it seemed to be causing
problems on OS X (https://github.com/facebook/proxygen/issues/3).

Test Plan: unit tests

Reviewed By: ldbrandy@fb.com

Subscribers: folly-diffs@, yfeldblum, chalfant

FB internal diff: D2129100

Signature: t1:2129100:1433458268:0b6b3696dde6c2bd13b89bc7ec58f0b1898be458

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