RFR: 8316190: Improve MemorySegment::toString

[Per Minborg]

This PR proposes to improve the MemorySegment::toString to reduce cluttering and add missing comas.

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Commit messages:
 - Improve MemorySegment::toString

Changes: https://git.openjdk.org/jdk/pull/15740/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15740&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8316190
  Stats: 26 lines in 3 files changed: 24 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/15740.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15740/head:pull/15740

PR: https://git.openjdk.org/jdk/pull/15740