Support assembly with more modern Java versions

[dievsky]

viktor can currently be assembled only with JDK 9 or earlier. That is because the JNI headers are generated using javah, which was removed in JDK 10. The headers are now supposed to be generated using javac -h, but this works only for Java source files, which we don't have, since our sources are in Kotlin. This is not optimal, since JDK 8 is fairly old.

Possible workarounds:

1. use a third-party javah stand-in, like gjavah;
2. use a javap workaround;
3. rewrite native definitions in Java (currently NativeSpeedups.kt and parts of Loader.kt);
4. wait until this issue is resolved by the Kotlin team.