RFR: 8315585: Optimization for decimal to string [v15]

[Shaojin Wen]

> I submitted PR #15555 before, and there were too many changes. I split it into multiple PRs with small changes. This one is one of them.
> 
> this PR removed the duplicate code for getChars in BigDecimal#StringBuilderHelper, i also make performance faster.
> Please review and don't hesitate to critique my approach and patch.

Shaojin Wen has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 19 commits:

 - Merge remote-tracking branch 'upstream/master' into optim_decimal_to_string_x1
 - Merge remote-tracking branch 'upstream/master' into optim_decimal_to_string_x1
 - Merge remote-tracking branch 'upstream/master' into optim_decimal_to_string_x1
 - revert use JLA
 - merge from master
 - Merge remote-tracking branch 'upstream/master' into optim_decimal_to_string_x1
   
   # Conflicts:
   #	src/java.base/share/classes/java/math/BigDecimal.java
 - use JLA
 - update copyright info & comments
 - reduce duplicate code & code format & refactor benchmark case
 - Merge remote-tracking branch 'upstream/master' into optim_decimal_to_string_x1
 - ... and 9 more: https://git.openjdk.org/jdk/compare/a474b372...fa5bddc7

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Changes: https://git.openjdk.org/jdk/pull/16006/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16006&range=14
  Stats: 464 lines in 3 files changed: 215 ins; 139 del; 110 mod
  Patch: https://git.openjdk.org/jdk/pull/16006.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16006/head:pull/16006

PR: https://git.openjdk.org/jdk/pull/16006