RFR: 8338158: Cleanup ShouldNotXXX uses in machnode.cpp

[Kim Barrett]

Please review this change to machnode.cpp to remove dead code following calls
to ShouldNotReachHere() and ShouldNotCallThis().  This takes advantage of the
availability and use of [[noreturn]] attributes on all supported platforms,
and makes all uses of those functions in this file consistent in this respect.

As a side effect, this removes some -Wzero-as-null-pointer-constant warnings.

Testing: mach5 tier1

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Commit messages:
 - ShouldNotXXX cleanup in machnode.cpp

Changes: https://git.openjdk.org/jdk/pull/20540/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20540&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338158
  Stats: 9 lines in 1 file changed: 0 ins; 4 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/20540.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20540/head:pull/20540

PR: https://git.openjdk.org/jdk/pull/20540