RFR: 8259862: MutableSpace's end should be atomic

[Kim Barrett]

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

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Commit messages:
 - make _end volatile and use atomic access

Changes: https://git.openjdk.java.net/jdk/pull/2323/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2323&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8259862
  Stats: 19 lines in 4 files changed: 4 ins; 9 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2323.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2323/head:pull/2323

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