Re-enable Scala-Native cross modules

[lihaoyi]

I disabled them due to

[705] [error] Found 1 unreachable symbols!
[705] [error] Unknown type java.lang.ProcessHandle, referenced from:
[705]          method os.SubProcess.destroy(long, bool, bool): scala.runtime.BoxedUnit at SubProcess.scala:192
[705]          method os.SubProcess.destroyForcibly(): scala.runtime.BoxedUnit at SubProcess.scala:197
[705]          method os.ProcessLike.join(long, long): bool at SubProcess.scala:75
[705]          method os.SubProcess.join(long, long): bool at SubProcess.scala:105
[705]          method os.proc.call(os.Path, scala.collection.immutable.Map, os.ProcessInput, os.ProcessOutput, os.ProcessOutput, bool, long, bool, bool, long, bool): os.CommandResult at ProcessOps.scala:227
[705]  private method test.os.SubprocessTests$.$anonfun$tests$66(): scala.util.Left at SubprocessTests.scala:239
[705]          method test.os.SubprocessTests$$$Lambda$71.apply(): java.lang.Object at SubprocessTests.scala:16
[705]  private method test.os.SubprocessTests$.$anonfun$tests$1(): scala.util.Right at SubprocessTests.scala:16
[705]          method test.os.SubprocessTests$$$Lambda$72.apply(): java.lang.Object at SubprocessTests.scala:16
[705]     constructor test.os.SubprocessTests$() at SubprocessTests.scala:16
[705]          method test.os.SubprocessTests$$SN$ReflectivelyInstantiate$.apply(): java.lang.Object at SubprocessTests.scala:11
[705] static constructor test.os.SubprocessTests$ at SubprocessTests.scala:11

Seems like Scala-Native is missing some classes and methods