RFR: 8234773: Fix ThreadsSMRSupport::_bootstrap_list

[Kim Barrett]

Please review this change to ThreadsList and the singleton _bootstrap_list.
ThreadsList is made noncopyable, with _bootstrap_list changed to be direct
initialized to avoid referencing the copy constructor.  The ThreadsList
constructor now uses a static array for the 0-entry case, so that static
ctor/dtor of _bootstrap_list doesn't involve C-heap allocation.

Testing:
mach5 tier1

Local (linux-x64) build with -fno-elide-constructors and ran hotspot:tier1.
This has a failure unrelated to this change:
compiler/intrinsics/klass/CastNullCheckDroppingsTest.java fails with
#  Internal Error (../../src/hotspot/share/jfr/utilities/jfrVersionSystem.inline.hpp:98), pid=31086, tid=31713
#  assert(node->_live) failed: invariant
Filed new bug: https://bugs.openjdk.java.net/browse/JDK-8259036

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Commit messages:
 - fix

Changes: https://git.openjdk.java.net/jdk/pull/1921/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1921&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8234773
  Stats: 25 lines in 2 files changed: 18 ins; 2 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1921.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1921/head:pull/1921

PR: https://git.openjdk.java.net/jdk/pull/1921