linuxkpi: Add timer64_to_tm()

The function converts a number of seconds since Epoch to a struct tm.
The implementation was copied from contrib/tzcode/localtime.c.

The amdgpu DRM driver started to use it in Linux 6.15.

Reviewed by: bz
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D58758