RFR: 8275863: Use encodeASCII for ASCII-compatible DoubleByte encodings

[Claes Redestad]

Enhance ASCII-compatible `DoubleByte` encodings to make use of the `StringCoding.implEncodeAsciiArray` intrinsic, which makes many such `CharsetEncoder`s encode ASCII text at speeds comparable to most single-byte encoders - and also more in line with how well these charsets behave when calling `String.getBytes`:

Before:

Benchmark                   (size)      (type)  Mode  Cnt   Score   Error  Units
CharsetEncodeDecode.encode   16384  ISO-8859-1  avgt   30   3.021 ± 0.120  us/op
CharsetEncodeDecode.encode   16384   Shift-JIS  avgt   30  47.793 ± 1.942  us/op
CharsetEncodeDecode.encode   16384      GB2312  avgt   30  49.598 ± 2.006  us/op
CharsetEncodeDecode.encode   16384      EUC-JP  avgt   30  68.709 ± 5.019  us/op
CharsetEncodeDecode.encode   16384      EUC-KR  avgt   30  48.033 ± 1.651  us/op


Patched:

Benchmark                   (size)      (type)  Mode  Cnt  Score   Error  Units
CharsetEncodeDecode.encode   16384  ISO-8859-1  avgt   30  2.856 ± 0.078  us/op
CharsetEncodeDecode.encode   16384   Shift-JIS  avgt   30  5.287 ± 0.209  us/op
CharsetEncodeDecode.encode   16384      GB2312  avgt   30  5.490 ± 0.251  us/op
CharsetEncodeDecode.encode   16384      EUC-JP  avgt   30  7.657 ± 0.368  us/op
CharsetEncodeDecode.encode   16384      EUC-KR  avgt   30  5.718 ± 0.267  us/op


The `isASCIICompatible` predicate on `DoubleByte` was added in JEP 254 for the purpose of implementing such ASCII fast-paths, but is only used in what is now `String.encodeWithEncoder` to speed up `String.getBytes(...)` operations.

Testing: tier1-3

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Commit messages:
 - Use field directly
 - Grant accessClassInPackage.jdk.internal.access permission to jdk.charsets module
 - 8275863: Use encodeASCII for ASCII-compatible DoubleByte encodings

Changes: https://git.openjdk.java.net/jdk/pull/6102/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6102&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8275863
  Stats: 35 lines in 5 files changed: 24 ins; 4 del; 7 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6102.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6102/head:pull/6102

PR: https://git.openjdk.java.net/jdk/pull/6102