LinuxKPI: Add x86_vendor field to struct cpuinfo_x86

and initialize it at linuxkpi module load.

Sponsored by: Serenity Cyber Security, LLC
Reviewed by: manu
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D42820

(cherry picked from commit b8c88a61750174f62db45784d6b4dc98de4073b1)