RFR: Copy of base64 micros from openjdk main for jdk8 comparisons

[Claes Redestad]

Include a few recently added Base64 micros to facilitate comparisons and tracking progress from JDK 8 and onwards.

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Commit messages:
 - Copy of base64 micros from openjdk main for jdk8 comparisons

Changes: https://git.openjdk.org/jmh-jdk-microbenchmarks/pull/12/files
 Webrev: https://webrevs.openjdk.org/?repo=jmh-jdk-microbenchmarks&pr=12&range=00
  Stats: 305 lines in 3 files changed: 305 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jmh-jdk-microbenchmarks/pull/12.diff
  Fetch: git fetch https://git.openjdk.org/jmh-jdk-microbenchmarks pull/12/head:pull/12

PR: https://git.openjdk.org/jmh-jdk-microbenchmarks/pull/12