teach clang to automatically realign x86 stacks when appropriate

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.

Start with the linked issue comment and investigate how Clang handles stack alignment for Android versions before 24 on x86, especially for constructors. Done means Clang automatically realigns the stack when those conditions apply without breaking the stated inline-assembly constraints.

Written by the indexing model from the issue text.