MDEV-19929 Add a startup message about PAUSE instruction timing

[dr-m]

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• [x] The Jira issue number for this PR is: MDEV-19929

Description

On IA-32 and AMD64 (x86-64) CPU implementations, the latency of the PAUSE instruction (REPZ NOP that was repurposed from a no-operation on the Intel Pentium 4) can vary a lot, and we may adjust the my_cpu_relax_multiplier accordingly. Let us display the value of that variable on startup if --log-warnings=3 or larger was specified.

Release Notes

On x86, if mariadbd is started up with --log-warnings=3 or larger, a message like this will appear in the server error log:

[Note] Using PAUSE multiplier 200

How can this PR be tested?

On an IA-32 or AMD64 based system:

./mtr --mysqld=-W3 main.1st
grep PAUSE var/log/mysqld.1.err

Basing the PR against the correct MariaDB version

• [ ] This is a new feature and the PR is based against the latest MariaDB development branch.
• [x] This is a bug fix and the PR is based against the earliest maintained branch in which the bug can be reproduced.

PR quality check

• [x] I checked the CODING_STANDARDS.md file and my PR conforms to this where appropriate.
• [ ] For any trivial modifications to the PR, I am ok with the reviewer making the changes themselves.

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