Integrated: 8286859: NullPointerException on extent-local variables

[Andrew Haley]

Fixes the problem by making the pointer from JavaThread into OOP storage not immutable

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Commit messages:
 - 8286859: NullPointerException on extent-local variables
 - Merge branch 'fibers' of https://github.com/openjdk/loom into fibers
 - Merge branch 'fibers' of https://github.com/openjdk/loom into fibers
 - Merge https://github.com/openjdk/loom into fibers
 - Merge branch 'fibers' of https://github.com/openjdk/loom into fibers
 - Merge https://github.com/openjdk/loom into fibers
 - Merge https://github.com/openjdk/loom into fibers
 - Merge https://github.com/openjdk/loom into fibers
 - Merge https://github.com/openjdk/loom into fibers
 - Merge https://github.com/openjdk/loom into fibers
 - ... and 20 more: https://git.openjdk.java.net/loom/compare/7167e5c7...842f1568

Changes: https://git.openjdk.java.net/loom/pull/187/files
 Webrev: https://webrevs.openjdk.java.net/?repo=loom&pr=187&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8286859
  Stats: 6 lines in 1 file changed: 4 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/loom/pull/187.diff
  Fetch: git fetch https://git.openjdk.java.net/loom pull/187/head:pull/187

PR: https://git.openjdk.java.net/loom/pull/187