RFR: 8277100: Dynamic dump can inadvertently overwrite default CDS archive

[Calvin Cheung]

Please review this small fix for preventing the default CDS archive from being overwritten via the
-XX:ArchiveClassesAtExit VM option.

If the user sets the -XX:ArchiveClassesAtExit option pointing to the default CDS archive, VM will
exit with the following message:

`Cannot specify the default CDS archive for -XX:ArchiveClassesAtExit: <path to classes.jsa>`

Passed CI tiers 1 and 2 tests.

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Commit messages:
 - 8277100: Dynamic dump can inadvertently overwrite default CDS archive

Changes: https://git.openjdk.java.net/jdk/pull/6896/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6896&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277100
  Stats: 63 lines in 2 files changed: 63 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6896.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6896/head:pull/6896

PR: https://git.openjdk.java.net/jdk/pull/6896