RFR: 7904122: Compilation of jasm files leads to class file version warning

[Jaikiran Pai]

Can I please get a review of this change which fixes the issue noted in https://bugs.openjdk.org/browse/CODETOOLS-7904122?

The commit here adds `-cv 52.0` as an option when launching the `jasm` tool for compiling `jasm` files through jtreg's `@compile` action. This class file version will be used as the default version if the jasm file doesn't explicitly specify a class file version in it. The use of "-cv" option prevents the warning from being issued.

An existing self test has been updated to verify this change.

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Commit messages:
 - 7904122: Compilation of jasm files leads to class file version warning

Changes: https://git.openjdk.org/jtreg/pull/303/files
  Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=303&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7904122
  Stats: 24 lines in 2 files changed: 15 ins; 0 del; 9 mod
  Patch: https://git.openjdk.org/jtreg/pull/303.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/303/head:pull/303

PR: https://git.openjdk.org/jtreg/pull/303