[URGENT] [8u] Re-cast JNI critical strings 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.

8u webrev:
   https://cr.openjdk.java.net/~shade/shenandoah/8u-jni-critical-strings/webrev.01/

The resulting difference against 8u upstream would be:
   https://cr.openjdk.java.net/~shade/shenandoah/8u-jni-critical-strings/8u-upstream.diff

Testing: hotspot_gc_shenandoah; minimal1 build (to test INCLUDE_ALL_GCS path)

-- 
Thanks,
-Aleksey