RFR: 8259862: MutableSpace's end should be atomic

[Thomas Schatzl]

On Sat, 30 Jan 2021 05:51:38 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

> Please review this change to MutableSpace, making its _end member volatile
> and using Atomic operations to access the _top and _end members.  Some
> unused accessor functions that would otherwise need updating are removed.
> 
> Testing:
> mach5 tier1

Marked as reviewed by tschatzl (Reviewer).

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PR: https://git.openjdk.java.net/jdk/pull/2323