Turns out I wasn't building the syscall libraries. Do so.

plat/linux/libsys/errno.s

@@ -0,0 +1,28 @@
+#
+! $Source: /cvsroot/tack/Ack/plat/linux386/libsys/errno.s,v $
+! $State: Exp $
+! $Revision: 1.1 $
+
+! Declare segments (the order is important).
+
+.sect .text
+.sect .rom
+.sect .data
+.sect .bss
+
+#define D(e) .define e; e
+
+.sect .data
+
+! Define various ACK error numbers. Note that these are *not* ANSI C
+! errnos, and are used for different purposes.
+
+D(ERANGE)         = 1
+D(ESET)           = 2
+D(EIDIVZ)         = 6
+D(EHEAP)          = 17
+D(EILLINS)        = 18
+D(EODDZ)          = 19
+D(ECASE)          = 20
+D(EBADMON)        = 25
+