Make vmkit usable from the OpenJDK 'java' launcher

[dtzWill]

• Build vmkit into libjvm.so, which means some engineering issues regarding -fPIC (particularly the assembly in nullpointerexception/stackoverflow handlers needs to be updated for this), but otherwise relatively minor.
• Implement JVM_CreateJavaVM and related, which implies an implementation of JVM_AttachCurrentThread. Given our current assertions about thread layout (the way we do TLS among other things) makes this more or less impossible since we don't control the creation of the thread we want to attach.

For now the 'j3' launcher works, but this would go a long way towards facilitating better integration with the OpenJDK tools and make vmkit more appealing to the community perhaps.