RFR: Merge jdk20

[Jesper Wilhelmsson]

Forwardport JDK 20 -> JDK 21

-------------

Commit messages:
 - Merge remote-tracking branch 'jdk20/master' into Merge_jdk20
 - 8297235: ZGC: assert(regs[i] != regs[j]) failed: Multiple uses of register: rax
 - 8298649: JFR: RemoteRecordingStream support for checkpoint event sizes beyond u4
 - 8298296: gc/TestFullGCCount.java fails with "System.gc collections miscounted."
 - 8294902: Undefined Behavior in C2 regalloc with null references
 - 8298425: System.console().readLine() hangs in jshell
 - 8298520: C2: assert(found_opaque == res) failed: wrong pattern
 - 8298527: Cygwin's uname -m returns different string than before
 - 8298700: Typo in DocTree comment
 - 8297305: Clarify that javax.lang.model.util.Elements.overrides is irreflexive
 - ... and 1 more: https://git.openjdk.org/jdk/compare/c05dbac3...873a7e3a

The webrevs contain the adjustments done while merging with regards to each parent branch:
 - master: https://webrevs.openjdk.org/?repo=jdk&pr=11678&range=00.0
 - jdk20: https://webrevs.openjdk.org/?repo=jdk&pr=11678&range=00.1

Changes: https://git.openjdk.org/jdk/pull/11678/files
  Stats: 216 lines in 17 files changed: 132 ins; 41 del; 43 mod
  Patch: https://git.openjdk.org/jdk/pull/11678.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11678/head:pull/11678

PR: https://git.openjdk.org/jdk/pull/11678