GooglePlay requires 64bit app

[KobayashiKeisuke]

https://android-developers.googleblog.com/2019/01/get-your-apps-ready-for-64-bit.html

In this August, Android Apps for Google Play are required 64bit.
However, this plugin has only 32bit version.

Are there any plans to prepare 64bit plugin?
or, do I have to build 64bit version from original repository (https://github.com/bulletphysics/bullet3) by myself ?

[Phong13]

Thanks for the heads up. I will build a 64 version and include that in the project.

[KobayashiKeisuke]

We need 64bit plugin on my job, so I fixed android makefile.
Phong13/BulletSharpPInvoke#2

If you would like it, I appreciate for merging this PR.
Thank you.