Defer unloader JNI libraries of boot classpath as late as possible.

Also don't try to lock at this point.
A lock would run a checkpoint on threads that are dying, and this can
lead to crashes, eg:
https://build.chromium.org/p/client.art/builders/host-x86-cms/builds/4412/steps/test%20debuggable/logs/stdio

+*** *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***
+Fatal signal 11 (SIGSEGV), code 1 (SEGV_MAPERR) fault addr (nil)
+OS: Linux 4.4.0-78-generic (x86_64)
+Cmdline: <unset>
+Thread: 10797 "Shutdown thread"
+Registers:
+    eax: 0x00000000    ebx: 0xf739c104    ecx: 0xf739f794    edx: 0x00000001
+    edi: 0x00000000    esi: 0x56748c54    ebp: 0xffd2bd18    esp: 0xffd2bcc0
+    eip: 0xf6c18c8c                    eflags: 0x00010246 [ PF ZF IF ]
+     cs: 0x00000023     ds: 0x0000002b     es: 0x0000002b     fs: 0x0000006b
+     gs: 0x00000063     ss: 0x0000002b
+Backtrace:
+	#00 pc 001ffc8c  /b/build/slave/host-x86-cms/build/out/host/linux-x86/lib/libartd.so (art::Thread::CheckEmptyCheckpointFromMutex()+428)
+	  art::Thread::ReadFlag(art::ThreadFlag) const
+	   art/runtime/thread.h:1089
+	  art::Thread::CheckEmptyCheckpointFromMutex()
+	   art/runtime/thread-inl.h:94
+	#01 pc 001fb40e  /b/build/slave/host-x86-cms/build/out/host/linux-x86/lib/libartd.so (art::Mutex::ExclusiveLock(art::Thread*)+462)
+	  art::Mutex::ExclusiveLock(art::Thread*)
+	   art/runtime/base/mutex.cc:382
+	#02 pc 0064d9b7  /b/build/slave/host-x86-cms/build/out/host/linux-x86/lib/libartd.so (art::Libraries::UnloadBootNativeLibraries(_JavaVM*)+103)
+	  MutexLock
+	   art/runtime/base/mutex.h:431
+	  art::Libraries::UnloadBootNativeLibraries(_JavaVM*)
+	   art/runtime/jni/java_vm_ext.cc:230

Test: 004-ThreadStress
Change-Id: I7f9b69590983ff4e3276a0ad1ef5c7453d692b04