RFR: 8317432: Async UL: Do not recompute strlen()

[Johan Sjölen]

Hi,

This change does three things:

1. At construction of message we used to compute `strlen` and then `strcpy`, we change this to do only one `strlen` and then a `memcpy` instead.
2. A `strlen` used to be required to calculate the size when finding the next message in the iterator. We instead store the previously computed message length and so the `strlen` is no longer necessary. The cost of this is 8 bytes extra per message (64 -> 72 bytes).
3. There's also some code which alters the alignment to explicitly handle the alignment of `Message` and ensure that `_pos` starts at a `Message`-aligned pointer.

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Commit messages:
 - Copyright
 - Ensure alignment of _pos on reset()
 - Copyright
 - Use alignof() and fix comments
 - Fix potential alignment issue
 - Store the message length in Message

Changes: https://git.openjdk.org/jdk/pull/16029/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16029&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8317432
  Stats: 24 lines in 2 files changed: 11 ins; 1 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/16029.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16029/head:pull/16029

PR: https://git.openjdk.org/jdk/pull/16029