teach clang to automatically realign x86 stacks when appropriate

[DanAlbert]

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https://github.com/android-ndk/ndk/issues/693#issuecomment-390815633

The longer term fix is to teach Clang to automatically realign the stack for Android < 24, x86, and when the function is a constructor. With that change the only way this will create a problem for people is if they're trying to use some inline assembly that can't satisfy these constraints specifically in a constructor.