Add x86 build

It allows to execute application at near native performance on Android emulator
when using x86 system image, KVM enabled and GPU Acceleration.

don't forget to add this dependency in your project's pom.xml :

        <dependency>
            <groupId>com.badlogicgames.gdx</groupId>
            <artifactId>gdx-platform</artifactId>
            <version>${gdx.version}</version>
            <classifier>natives-x86</classifier>
            <scope>provided</scope>
        </dependency>