RFR: 8364128: Improve gathering of cpu feature names using FormatBuffer [v2]

[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 three additional commits since the last revision:

 - Add changes missed in vm_version_bsd_aarch64.cpp
   
   Signed-off-by: Ashutosh Mehra <asmehra at redhat.com>
 - Revert FormatBuffer changes
   
   Signed-off-by: Ashutosh Mehra <asmehra at redhat.com>
 - Use stringStream instead of FormatBuffer
   
   Signed-off-by: Ashutosh Mehra <asmehra at redhat.com>

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/26515/files
  - new: https://git.openjdk.org/jdk/pull/26515/files/704b12c4..6ab46995

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26515&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26515&range=00-01

  Stats: 136 lines in 10 files changed: 25 ins; 56 del; 55 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