[jvm] ExceptionInInitializerError

I installed haxe 4.3.1 for a project that previously used haxe 4.3.0-rc.1+966864c. It works well on every target but java, where it throws the exception below. Haxe 4.3.0 has the same problem too.

Unfortunately I am not able to reproduce the error by means of a simple test, but the basic execution flow is trivial: I create an instance of `hx.concurrent.executor.Executor` in a static variable and, when I try to call the method `submit` on it, I get `ExceptionInInitializerError`.

I already contacted the author of the library haxe-concurrent and his opinion is that the exception is due to a bug of the jvm target.

```
java.lang.ExceptionInInitializerError
at utest.Test.delay(test-util/utest/Test.hx:152)
at utest.Runner.runCurrent(test-util/utest/Runner.hx:129)
at utest.Runner.gotoFirstTest(test-util/utest/Runner.hx:85)
at utest.Runner$Closure_evtRun_0.invoke(test-util/utest/Runner.hx:60)
at utest.Runner$Closure_evtRun_0.invoke(test-util/utest/Runner.hx)
at hx.concurrent.lock.AbstractAcquirable.execute(/Users/acarioni/haxe/haxe_libraries/haxe-concurrent/5.1.3/haxelib/src/hx/concurrent/lock/Acquirable.hx:55)
at utest.Runner.evtRun(test-util/utest/Runner.hx:55)
at utest.Runner$Closure_run_0.invoke(test-util/utest/Runner.hx:52)
at utest.Runner$Closure_run_0.invoke(test-util/utest/Runner.hx)
at hx.concurrent.lock.AbstractAcquirable.execute(/Users/acarioni/haxe/haxe_libraries/haxe-concurrent/5.1.3/haxelib/src/hx/concurrent/lock/Acquirable.hx:55)
at utest.Runner.run(test-util/utest/Runner.hx:50)
at haxe.root.TestAll.main(test/TestAll.hx:93)
at haxe.root.TestAll.main(test/TestAll.hx:1)
Caused by: java.lang.ClassCastException: class hx.concurrent.thread.ThreadPool$Closure_onStart_0 cannot be cast to class java.lang.Runnable (hx.concurrent.thread.ThreadPool$Closure_onStart_0 is in unnamed module of loader 'app'; java.lang.Runnable is in module java.base of loader 'bootstrap')
at sys.thread.Thread$NativeHaxeThread.(/Users/acarioni/haxe/versions/4.3.1/std/java/_std/sys/thread/Thread.hx:160)
at sys.thread.Thread$HaxeThread.create(/Users/acarioni/haxe/versions/4.3.1/std/java/_std/sys/thread/Thread.hx:104)
at hx.concurrent.thread.ThreadPool.onStart(/Users/acarioni/haxe/haxe_libraries/haxe-concurrent/5.1.3/haxelib/src/hx/concurrent/thread/ThreadPool.hx:102)
at hx.concurrent.ServiceBase$Closure_start_0.invoke(/Users/acarioni/haxe/haxe_libraries/haxe-concurrent/5.1.3/haxelib/src/hx/concurrent/Service.hx:65)
at hx.concurrent.ServiceBase$Closure_start_0.invoke(/Users/acarioni/haxe/haxe_libraries/haxe-concurrent/5.1.3/haxelib/src/hx/concurrent/Service.hx)
at hx.concurrent.lock.AbstractAcquirable.execute(/Users/acarioni/haxe/haxe_libraries/haxe-concurrent/5.1.3/haxelib/src/hx/concurrent/lock/Acquirable.hx:55)
at hx.concurrent.ServiceBase.start(/Users/acarioni/haxe/haxe_libraries/haxe-concurrent/5.1.3/haxelib/src/hx/concurrent/Service.hx:58)
at hx.concurrent.thread.ThreadPool.(/Users/acarioni/haxe/haxe_libraries/haxe-concurrent/5.1.3/haxelib/src/hx/concurrent/thread/ThreadPool.hx:61)
at hx.concurrent.executor.ThreadPoolExecutor.(/Users/acarioni/haxe/haxe_libraries/haxe-concurrent/5.1.3/haxelib/src/hx/concurrent/executor/ThreadPoolExecutor.hx:41)
at hx.concurrent.executor.Executor.create(/Users/acarioni/haxe/haxe_libraries/haxe-concurrent/5.1.3/haxelib/src/hx/concurrent/executor/Executor.hx:36)
at utest.Test$Test_Fields_.(test-util/utest/Test.hx:10)
... 13 more

```