RFR: 8364128: Improve gathering of cpu feature names using FormatBuffer [v3]

[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].
> Platforms affected: x86-64 and aarch64
> Other platforms can be done if and when Leyden changes are ported to them.
> 
> [0] https://github.com/openjdk/leyden/pull/84

Ashutosh Mehra has updated the pull request incrementally with one additional commit since the last revision:

  Fix compile failures
  
  Signed-off-by: Ashutosh Mehra <asmehra at redhat.com>

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Changes:
  - all: https://git.openjdk.org/jdk/pull/26515/files
  - new: https://git.openjdk.org/jdk/pull/26515/files/6ab46995..f3c80d7f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26515&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26515&range=01-02

  Stats: 8 lines in 4 files changed: 5 ins; 1 del; 2 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