util: Embed the JNI library (.so) in the jar file for the gem5 ops.

util: Embed the JNI library (.so) in the jar file for the gem5 ops.

This makes it easier to manage the java wrapper since there's only one
file. This change also splits up the command builder which builds the
java jar since we need to run one step which produces the .h, then a
second step to build the library, and then finally the step that
produces the jar. The first step is left as a command builder since the
scons Java builder still doesn't know about the -h option, but the
second step now uses the Jar builder.

Change-Id: I76e2e5e86bb3153f2ba69a75ad94cd4e065cd33d
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/28183
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>