Replace javah with javac

[dsamersoff]

Warning! These changes has an impact to the process.

1. It depends to mx changes (Generate native headers during java compilation graalvm/mx#142).
2. It's not clear to me how we should handle jdk8 that stay with javah

[dougxc]

@rschatz , does this look like the right way to accomodate the removal of javah from recent JDKs?

[rschatz]

It's not clear to me how we should handle jdk8 that stay with javah

That's not really a problem, javac on jdk8 does support the -h option. The question is how to support building with ecj/eclipse/netbeans/... I'll look into that.

[rschatz]

I made a new PR for mx (graalvm/mx#143) that works around the ecj build issue by still using javah there. With that, this PR should work. We have to wait until the mx change is merged, then I can merge this PR.

Thanks for the contribution.