RFR: 8364128: Improve gathering of cpu feature names using FormatBuffer

[Ashutosh Mehra]

This PR implements the code for cpu features names string using FormatBuffer. It also improves and extends FormatBuffer with an additional method that appends comma separated strings to the buffer. This is useful in creating the cpu features names string.
This code will also be useful in Leyden to implement cpu feature check for AOTCodeCache [0]

[0] https://github.com/openjdk/leyden/pull/84

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Commit messages:
 - 8364128: Improve gathering of cpu feature names using FormatBuffer

Changes: https://git.openjdk.org/jdk/pull/26515/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26515&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8364128
  Stats: 194 lines in 10 files changed: 97 ins; 29 del; 68 mod
  Patch: https://git.openjdk.org/jdk/pull/26515.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26515/head:pull/26515

PR: https://git.openjdk.org/jdk/pull/26515