RFR: 8370227: Migrate micros-javac benchmarks from jmh-jdk-microbenchmarks [v3]

[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` in the `micros-javac` folder, similar to what you would do today in the jmh-jdk-microbenchmarks project.

Claes Redestad has updated the pull request incrementally with two additional commits since the last revision:

 - Tabs not allowed
 - Missing comma

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Changes:
  - all: https://git.openjdk.org/jdk/pull/27917/files
  - new: https://git.openjdk.org/jdk/pull/27917/files/b3caf9d8..9a6d4bf8

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27917&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27917&range=01-02

  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 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