linuxkpi: Add get_unaligned_le64()

This function was the only one missing in the get_unaligned_*()
family. This is going to be used by the imported linux_siphash.c in a
future commit, which itself is used by DRM drivers starting from Linux
6.10.

Reviewed by: bz
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D54500