[URGENT] [8u] Re-cast MetadataOnStack patch to be Shenandoah-specific

[Aleksey Shipilev]

There is a hard requirement for 8u-aarch64 integration that any non-mainline patch in shared code 
was either trivial, or be obviously Shenandoah-specific. Unfortunately, at least one of the changes 
affects the shared path. With this patch, I demote it to Shenandoah-specific paths only.

This blocks 8u integration.

Note: a similar problem exists in jni.cpp, see: 
https://builds.shipilev.net/patch-openjdk-shenandoah-jdk8/hotspot/src/share/vm/prims/jni.cpp.sdiff.html 
-- I'll take a stab at it, but Zhengyu, please take a look as well. That one is urgent as well.

8u webrev:
   https://cr.openjdk.java.net/~shade/shenandoah/8u-metadata-on-stack/webrev.01/

Testing: hotspot_gc_shenandoah

-- 
Thanks,
-Aleksey