JIT JNI stubs.

Allow the JIT compiler to compile JNI stubs and make sure
they can be collected once they are not in use anymore.

Test: 667-jit-jni-stub
Test: Pixel 2 XL boots.
Test: m test-art-host-gtest
Test: testrunner.py --host --jit
Test: testrunner.py --target --jit
Bug: 65574695
Change-Id: Idf81f50bcfa68c0c403ad2b49058be62b21b7b1f