RFR: 8271840: Add simple Integer.toString microbenchmarks

[Claes Redestad]

This adds Integer.toString microbenchmarks, roughly similar to the Long.toString microbenchmarks (toStringSmall should use an equivalent data set). This is useful for comparison purposes and completeness.

Also tuned existing benchmarks to harmonize setup, reduce runtime without significantly harming reliability of results, and remove the explicit Threads.MAX from Longs

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Commit messages:
 - Add simple Integer.toString microbenchmarks

Changes: https://git.openjdk.java.net/jdk/pull/4988/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4988&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8271840
  Stats: 43 lines in 2 files changed: 33 ins; 3 del; 7 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4988.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4988/head:pull/4988

PR: https://git.openjdk.java.net/jdk/pull/4988