Revert "Revert "JIT JNI stubs.""

The original CL,
    https://android-review.googlesource.com/513417 ,
had a bug for class unloading where a read barrier was
executed at the wrong time from
    ConcurrentCopying::MarkingPhase() ->
    ClassLinker::CleanupClassLoaders() ->
    ClassLinker::DeleteClassLoader() ->
    JitCodeCache::RemoveMethodsIn() ->
    JitCodeCache::JniStubKey::UpdateShorty() ->
    ArtMethod::GetShorty().
This has been fixed by removing sources of the read barrier
from ArtMethod::GetShorty().

Test: testrunner.py --host --prebuild --jit --no-relocate \
      --no-image -t 998-redefine-use-after-free
Bug: 65574695
Bug: 69843562

This reverts commit 47d31853.

Change-Id: I06e7a15b09d9ff11cde15a7d1529644bfeca15e0