MDEV-38332 Detect Hurd based on CMAKE_SYSTEM_NAME being "GNU"

[ottok]

• The Jira issue number for this PR is: MDEV-38332

Description (=git commit message)

MDEV-38332 Detect Hurd based on CMAKE_SYSTEM_NAME being "GNU"

In 22afd13 Hurd detection was introduced using the processor type,
as incidentally only the hurd-i386 build on Debian was using i686.
This method however does not detect a hurd-amd64 system. Fix this
by using the actual system name following CMake conventions.

Hurd is a kernel project by the GNU project, thus system is called
"GNU".

Release Notes

This helps the hurd-amd64 build, but does not yet solve it, so better not mention in release notes.

How can this PR be tested?

Build on a Hurd system or see log at https://buildd.debian.org/status/fetch.php?pkg=mariadb&arch=hurd-amd64&ver=1%3A11.8.5-3%7Eexp3&stamp=1765605419&raw=0

Basing the PR against the correct MariaDB version

• This is a bug fix, and the PR is based against the earliest maintained branch in which the bug can be reproduced.

Commit 22afd13e7a13 was made on 10.11, so basing this fix on that branch.

PR quality check

• 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.