auth_socket does not build on hurd-amd64

This is a follow-up to MDEV-8535 that was fixed in 22afd13e7a13 by danblack

  # No hurd implementation

  IF(NOT CMAKE_SYSTEM_PROCESSOR STREQUAL "i686-AT386")

    SET(PLUGIN_AUTH_SOCKET YES CACHE STRING "")

  ENDIF()

This is generic for i686 but applies for hurd-i386 only as the Debian i386 builds run on x86_64 hosts. Compare

Linux i386 https://buildd.debian.org/status/fetch.php?pkg=mariadb&arch=i386&ver=1%3A11.8.5-3%7Eexp2&stamp=1765521421&raw=0
Kernel: Linux 6.12.57+deb13-amd64 #1 SMP PREEMPT_DYNAMIC Debian 6.12.57-1 (2025-11-05) amd64 (x86_64)

Hurd i386 https://buildd.debian.org/status/fetch.php?pkg=mariadb&arch=hurd-i386&ver=1%3A11.8.5-2%2Bb2&stamp=1765351617&raw=0
Kernel: GNU 0.9 GNU-Mach 1.8+git20250731-up-486-dbg/Hurd-0.9 hurd-i386 (i686-AT386)

However, I would like to have this trick work for hurd-amd64 too, which has:
https://buildd.debian.org/status/fetch.php?pkg=mariadb&arch=hurd-amd64&ver=1%3A11.8.5-3%7Eexp2&stamp=1765539272&raw=0
Kernel: GNU 0.9 GNU-Mach 1.8+git20250731-up-amd64-dbg/Hurd-0.9 hurd-amd64 (x86_64)

I guess we need to find a new CMake variable to make the `if` on..