[sh/jdk11u]: JVMTI related backports

[Zhengyu Gu]

I would like to backport these two JVMTI heap walk related backports:

8237396: JvmtiTagMap::weak_oops_do() should not trigger barriers
8237632: Shenandoah: accept NULL fwdptr to cooperate with JVMTI and JFR

The backports clear JVMTI related test failures.

Webrev: 
http://cr.openjdk.java.net/~zgu/shenandoah/jvmti_sh_jdk11u/webrev.00/

Test:
   hotspot_gc_shenandoah
   vmTestbase_nsk_jvmti
   vmTestbase_nsk_jdi

Thanks,

-Zhengyu