RFR (XS) 8237080: fatal error: VM thread could block on lock that may be held by a JavaThread during safepoint: SharedDecoder_lock

[coleen.phillimore at oracle.com]

Summary: Set allow_vm_block to true for this lock.  It's 
_safepoint_check_never so it's sort of implied (you can't safepoint 
holding the lock and block out the vm thread).

open webrev at http://cr.openjdk.java.net/~coleenp/2020/8237080.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8237080

Ran test case with hard-coded failure, and tier1-6.

Thanks,
Coleen