RFR: 8370227: Migrate micros-javac benchmarks from jmh-jdk-microbenchmarks

[Claes Redestad]

To improve and consolidate workflow this PR proposes moving micros-javac from https://github.com/openjdk/jmh-jdk-microbenchmarks into the main OpenJDK tree.

Most of jmh-jdk-microbenchmarks was effectively migrated in-tree as part of https://openjdk.org/jeps/230 
These javac benchmarks, however, were added after migration as a stop-gap solution: they rely on an antrun task to download a configurable set of JDK source files deemed problematic to integrate with the JDK build and the resulting benchmark jar is quite bulky (~52M), so unconditionally piling that on the test image built on every change seems unwise.

Proposal:
- The JMH benchmarks are moved over as-is along with a flattened and updated maven pom.xml into a new non-jtreg test root, `test/benchmarks/micros-javac`
- Any integration with the JDK build is deferred to a future enhancement. I.e., no integration with `make test` or equivalent of `make micros-javac` for now. Instead users of these benchmarks are encouraged to build the benchmarks by running `mvn install` on the , as you would the benchmarks out of the jmh-jdk-microbenchmarks project.

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Commit messages:
 - 8370227: Migrate micros-javac benchmarks from jmh-jdk-microbenchmarks

Changes: https://git.openjdk.org/jdk/pull/27917/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27917&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8370227
  Stats: 631 lines in 5 files changed: 631 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/27917.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27917/head:pull/27917

PR: https://git.openjdk.org/jdk/pull/27917