RFR: 8370013: Refactor Double.toHexString to eliminate regex and StringBuilder [v5]

[Shaojin Wen]

> This PR refactors the Double.toHexString method to improve its performance by eliminating the use of StringBuilder and regex operations. The new implementation uses a direct byte array approach to construct the hex string representation, which avoids the overhead of StringBuilder operations and regex pattern matching.
> 
> Existing tests in `java/lang/Double/ToHexString.java`.

Shaojin Wen has updated the pull request incrementally with one additional commit since the last revision:

  code style, from @rgiulietti

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Changes:
  - all: https://git.openjdk.org/jdk/pull/27811/files
  - new: https://git.openjdk.org/jdk/pull/27811/files/1631bec9..69fbee0e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27811&range=04
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27811&range=03-04

  Stats: 63 lines in 1 file changed: 1 ins; 4 del; 58 mod
  Patch: https://git.openjdk.org/jdk/pull/27811.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27811/head:pull/27811

PR: https://git.openjdk.org/jdk/pull/27811