net-mgmt/ipgen: fix build on non-amd64, 15-CURRENT

• 15-CURRENT ditches the CLLADDR macro; replace it with LLADDR
• cpu_in_cksum.S is mediocre assembly code that only works on amd64. Disable its use; the compiler will do just as good of a job.
• assume we always have atomic_* functions on FreeBSD, even if they are not macros (as is the case on armv7)

Approved by: portmgr (build fix blanket)
MFH: 2025Q2
See also: D50065