Reuse the StringUTF16::putCharsSB method instead of the Intrinsic in the StringUTF16::toBytes

[wenshao]

In the discussion of `8355177: Speed up StringBuilder::append(char[]) via Unsafe::copyMemory` (https://github.com/openjdk/jdk/pull/24773 <https://github.com/openjdk/jdk/pull/24773 >), @liach (Chen Liang) suggested reusing the StringUTF16::putCharsSB method introduced in PR #24773 instead of the Intrinsic implementation in the StringUTF16::toBytes method.
Original:
```java
 @IntrinsicCandidate
 public static byte[] toBytes(char[] value, int off, int len) {
 byte[] val = newBytesFor(len);
 for (int i = 0; i < len; i++) {
 putChar(val, i, value[off]);
 off++;
 }
 return val;
 }
```
After:
```java
 public static byte[] toBytes(char[] value, int off, int len) {
 byte[] val = (byte[]) Unsafe.getUnsafe().allocateUninitializedArray(byte.class, newBytesLength(len));
 putCharsSB(val, 0, value, off, off + len);
 return val;
 }
```
This replacement does not degrade performance. Running StringConstructor.newStringFromCharsMixedBegin verified that performance is consistent with the original on x64 and slightly improved on aarch64.
The implementation after replacing the Intrinsic implementation removed 100 lines of C++ code, leaving only Java and Unsafe code, no Intrinsic or C++ code, which makes the code more maintainable.
I've submitted a draft PR https://github.com/openjdk/jdk/pull/26553 <https://github.com/openjdk/jdk/pull/26553 > , please give me some feedback.
-
Shaojin Wen
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