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

[Roman Kennke]

> 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

Looks good, thank you!
Roman