RFR/RFC [8u]: Enable weak JNI handling with Shenandoah after JDK-8176100

[Aleksey Shipilev]

In Shenandoah/jdk8u code, we have a workaround for weak JNI handling:
  http://hg.openjdk.java.net/shenandoah/jdk8/hotspot/rev/119e9a5b24d5
  http://hg.openjdk.java.net/shenandoah/jdk8/hotspot/rev/d305c31da9f5

This workaround is not necessary anymore, after we got the backports of JDK-8176100:
  http://hg.openjdk.java.net/shenandoah/jdk8/hotspot/rev/32bc598624bd
  http://hg.openjdk.java.net/shenandoah/jdk8/hotspot/rev/327f9583546e

So, this changeset rolls back the workaround and injects Shenandoah calls where appropriate.

8u candidate fix:
  http://cr.openjdk.java.net/~shade/shenandoah/8u-enable-weak-jni/webrev.01/

Note aarch64 fails! This might mean the upstream patch is incomplete. And I cannot debug it properly
without having working aarch64 machine.

Testing: hotspot_gc_shenandoah (includes gc/shenandoah/jni/TestJNIGlobalRefs.sh
test that fails without the workaround, and passes again without the workaround _and_ the fixes in
platform-specific code); tested with x86_64

-- 
Thanks,
-Aleksey