Refactor Double.toHexString to eliminate regex and StringBuilder

[wenshao]

Referencing jddarcy's 8369312: Refactor Float.toHexString() to avoid use of regex (https://github.com/openjdk/jdk/pull/27780 <https://github.com/openjdk/jdk/pull/27780 >), we can further eliminate the regex and StringBuilder in Double.toHexString to improve performance.
To make my proposal easier to understand, I've prepared a draft PR: https://github.com/openjdk/jdk/pull/27811 <https://github.com/openjdk/jdk/pull/27811 >
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Shaojin Wen
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