RFR: 8323832: Load JVMCI with the platform class loader [v2]

[Doug Simon]

> This PR changes `jdk.internal.vm.ci` such that it is loaded by the platform class loader instead of the boot class loader. This allows Native Image to load a version of JVMCI different than the version on top of which Native Image is running. This capability is demonstrated and tested by `LoadAlternativeJVMCI.java`.

Doug Simon has updated the pull request incrementally with one additional commit since the last revision:

  use null to denote boot class loader as delegation parent

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Changes:
  - all: https://git.openjdk.org/jdk/pull/17520/files
  - new: https://git.openjdk.org/jdk/pull/17520/files/e7d5801a..1642276e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17520&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17520&range=00-01

  Stats: 8 lines in 1 file changed: 1 ins; 7 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/17520.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17520/head:pull/17520

PR: https://git.openjdk.org/jdk/pull/17520