RFR: Scope locals cleanups

[Andrew Haley]

Scope locals cleanups

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Commit messages:
 - Merge from fibers.
 - Merge https://github.com/openjdk/loom into fibers
 - Add system property "java.lang.ScopeLocal.PRESERVE_SCOPE_LOCAL_CACHE"
 - Update tests.
 - 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 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
 - ... and 15 more: https://git.openjdk.java.net/loom/compare/417f271d...13919909

Changes: https://git.openjdk.java.net/loom/pull/84/files
 Webrev: https://webrevs.openjdk.java.net/?repo=loom&pr=84&range=00
  Stats: 98 lines in 4 files changed: 37 ins; 46 del; 15 mod
  Patch: https://git.openjdk.java.net/loom/pull/84.diff
  Fetch: git fetch https://git.openjdk.java.net/loom pull/84/head:pull/84

PR: https://git.openjdk.java.net/loom/pull/84