Hmmm, it seems there was also a compile-time warning on locore.S which was not treated as fatal...
In file included from ./xen/xen-public/arch-x86/xen.h:61:0,
from ./xen/xen-public/xen.h:34,
from /build/netbsd-compat/src/sys/arch/amd64/amd64/locore.S:280:
./xen/xen-public/arch-x86/xen-x86_64.h:58:0: warning: "FLAT_RING3_CS64" redefined
#define FLAT_RING3_CS64 0xe033 /* GDT index 261 */
In file included from /build/netbsd-compat/src/sys/arch/amd64/amd64/locore.S:164:0:
./assym.h:177:0: note: this is the location of the previous definition
#define FLAT_RING3_CS64 57395