LinuxKPI: add put_unaligned_le16() and get_unaligned_be64()

Add the two new functions needed by wireless drivers by the same
implementation pattern we did for different sizes.

Sponsored by: The FreeBSD Foundation
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D40174

(cherry picked from commit ae1e336e05171b80123bf6db7457f19c174accc4)