handle emscripten in detail::linux_syscall
handle emscripten in detail::linux_syscall

Summary: Emscripten is another platform, like Windows, that does not have anything comparable to linux syscall.

Reviewed By: Gownta

Differential Revision: D35985405

fbshipit-source-id: ab132aca14407060a86bd1f3746c5334b45162b2

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly