RFR: 8343140: JfrJavaSupport uses the wrong accessors for sub-int fields

[Daniel Jeliński]

This test fixes the accessor used by `read_boolean_field` to access boolean fields of the `jdk.jfr.internal.dcmd.Argument` record. It also fixes the read accessors for `short` and `char` types, and marks the write accessors for `boolean`, `short` and `char` as unimplemented.

The existing accessor reads the boolean field as int. This results in unaligned access, and probably reads incorrect value on big-endian CPUs.

No new tests. I'm sure a test would be useful, but I have no idea how to write one. Existing tier1-3 tests continue to pass.

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Commit messages:
 - Use correct accessors

Changes: https://git.openjdk.org/jdk/pull/21952/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21952&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343140
  Stats: 8 lines in 1 file changed: 8 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/21952.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21952/head:pull/21952

PR: https://git.openjdk.org/jdk/pull/21952