RFR: 8290894: Reduce runtime of vm.lang microbenchmarks

[Eric Caspole]

Adds fork and iterations controls. This reduces the runtime from 4h26m to about 21m and still seems stable enough for weekly testing.

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Commit messages:
 - 8290894: Reduce runtime of vm.lang microbenchmarks

Changes: https://git.openjdk.org/jdk/pull/9619/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9619&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8290894
  Stats: 39 lines in 5 files changed: 30 ins; 3 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/9619.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9619/head:pull/9619

PR: https://git.openjdk.org/jdk/pull/9619