Fix Windows binary: statically link libstdc++
Fix Windows binary: statically link libstdc++

Summary:
The Windows binary we distribute is compiled with mingw64. The addition of C++ code in v0.143.0 caused us to start dynamically linking `libstdc++-6.dll`. this requires users to have mingw64 installed (not acceptable), or for us to package that DLL next to `flow.exe`. That's possible in theory but I couldn't get it to work correctly, and it's nice for the binary to be self-contained.

So we statically link `libstdc++` and `pthread` instead. The linker on Windows, `flexlink` from [flexdll](https://github.com/alainfrisch/flexdll), doesn't support `-static` or `-static-libstdc++` and [prefers](https://github.com/alainfrisch/flexdll/blob/1ba65c105ecf9976a663d67170b746cf3e679aa5/reloc.ml#L258-L268) the `.dll.a` dynamic interface over the `.a` static one, so I'm taking advantage of/abusing its search algorithm to resolve to `libstdc++-6.a` before `libstdc++-6.dll.a` by passing `-lstdc++.a` instead of `-lstdc++`.

Fixes https://github.com/facebook/flow/issues/8574

Pull Request resolved: https://github.com/facebook/flow/pull/8578

Reviewed By: dsainati1

Differential Revision: D26018528

Pulled By: mroch

fbshipit-source-id: bf43c1452056a0d045c9b7a09feb2804efac12f2

mroch/flow,mroch/flow,facebook/flow,mroch/flow,mroch/flow,mroch/flow,facebook/flow,mroch/flow,mroch/flow,facebook/flow,facebook/flow,facebook/flow,facebook/flow,facebook/flow