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

[Aleksey Shipilev]

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

-- 
Thanks,
-Aleksey