RFR: 8321467: MemorySegment.setString(long, String, Charset) throws IAE(Misaligned access)

[Maurizio Cimadamore]

This PR fixes a couple of aligned accesses when reading/writing strings. Such aligned accesses crept in when we optimized string read/write operations to work in bulk. As a result, depending on the maximum alignment constraints supported by the heap segment, some string operations might fail.

I've added some tests to make sure that all operations work as expected with unaligned semantics.

Note: I've considered inferring an alignment constraint from the provided charset, and then use aligned operations (and document that behavior), but I found that to be unsatisfactory: memory operations typically accept a layout, which allow clients to opt out from alignment checks if needed. But if we always infer an alignment constraint from the provided charset, clients would find themselves w/o an escape hatch. For this reason, I think the best way to fix this is to use unaligned operations when reading/writing the string.

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Commit messages:
 - Initial push

Changes: https://git.openjdk.org/jdk/pull/16999/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16999&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8321467
  Stats: 79 lines in 2 files changed: 69 ins; 6 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/16999.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16999/head:pull/16999

PR: https://git.openjdk.org/jdk/pull/16999