[8u] Bulk backports to sh/jdk8, 2020-12-01

[Roman Kennke]

> https://cr.openjdk.java.net/~shade/shenandoah/backports/jdk8u-20201201/webrev.8u.01 
> 
> 
> This backports the following batch of changes:
> 
>   [backport] 8253778: ShenandoahSafepoint::is_at_shenandoah_safepoint 
> should not access VMThread state from other threads
>   [backport] 8233574: Shenandoah: build is broken without jfr
>   [backport] 8255457: Shenandoah: cleanup ShenandoahMarkTask
>   [backport] 8255760: Shenandoah: match constants style in 
> ShenandoahMarkTask fallback
> 
> Testing: {x86_64,x86_32} hotspot_gc_shenandoah

Looks good!

Thanks!
Roman