NetBSD-current has src/sys/compat/linux/arch/amd64, a couple of which
(AFAICT) are identical apart from copyright and style, are acutally
shared in Linux, and thus arguably should be shared by our Linux
emulations.

Is there some reason to _not_ share, for example linux_socket.h?

But on the gripping hand is that I see no amd64 emulation for Linux in
NetBSD-3. What would be needed to add such support over and above adding
src/sys/compat/linux/arch/amd64, and suitable entries to arch/amd64/conf/GENERIC?

Is there any real downside to doing that, or requesting pullups (e.g.,
so that any skew in kernel APIs beteen -current and NetBSD-3 can be remediated)?