RFR: 8299324: inline_native_setCurrentThread lacks GC barrier for Shenandoah

[William Kemper]

Allow Shenandoah barrier to emit the store barrier for native memory. I believe it is safe to delete the assert on L202 because `obj` is not used here. Tested with `hotspot:hotspot_gc` and `hotspot:loom` with `JAVA_OPTIONS=-XX:+UseShenandoahGC` (and again with  -XX:TieredStopAtLevel=1).

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Commit messages:
 - Merge jdk:master into native-store-barrier
 - 8299324: inline_native_setCurrentThread lacks GC barrier for Shenandoah

Changes: https://git.openjdk.org/jdk/pull/12300/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12300&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8299324
  Stats: 5 lines in 1 file changed: 0 ins; 4 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/12300.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12300/head:pull/12300

PR: https://git.openjdk.org/jdk/pull/12300