[sh/jdk8u] Bulk backports to sh/jdk8

[Aleksey Shipilev]

Hi,

I would like to backport these simple things to reduce the difference between codebases:
   [backport] 8261413: Shenandoah: Disable class-unloading in I-U mode
   [backport] 8261251: Shenandoah: Use object size for full GC humongous
   [backport] 8259954: gc/shenandoah/mxbeans tests fail with -Xcomp
   [backport] 8259580: Shenandoah: uninitialized label in VerifyThreadGCState

Webrev:
   https://cr.openjdk.java.net/~shade/shenandoah/backports/jdk8u-20210531/webrev.01/

Testing: hotspot_gc_shenandoah {fastdebug,release}

-- 
Thanks,
-Aleksey