RFR: 8378698: Optimize Base64.Encoder#encodeToString

[Patrick Strawderman]

Avoid a byte array copy in encodeToString by using JavaLangAccess#uncheckedNewStringWithLatin1Bytes

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Commit messages:
 - 8378698: Optimize Base64.Encoder#encodeToString

Changes: https://git.openjdk.org/jdk/pull/29920/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29920&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8378698
  Stats: 5 lines in 1 file changed: 3 ins; 1 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/29920.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29920/head:pull/29920

PR: https://git.openjdk.org/jdk/pull/29920