[8u] Bulk backports to sh/jdk8, 2020-12-04

[Aleksey Shipilev]

https://cr.openjdk.java.net/~shade/shenandoah/backports/jdk8u-20201204/webrev.01/

This backports the following batch of changes:

  [backport] 8253778: ShenandoahSafepoint::is_at_shenandoah_safepoint should not access VMThread 
state from other threads
  [backport] 8257641: Shenandoah: Query is_at_shenandoah_safepoint() from control thread should 
return false

The second one fixes the regression from the first one.

Testing: {x86_64,x86_32} hotspot_gc_shenandoah

-- 
Thanks,
-Aleksey