RFR: 8317432: Async UL: Do not recompute strlen() [v2]

[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.

Johan Sjölen has updated the pull request incrementally with one additional commit since the last revision:

  Do not save the message length

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/16029/files
  - new: https://git.openjdk.org/jdk/pull/16029/files/e37f281f..a4257897

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16029&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16029&range=00-01

  Stats: 9 lines in 2 files changed: 0 ins; 2 del; 7 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