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

plat/linux/libsys/sbrk.c

@@ -0,0 +1,39 @@
+/* $Source$
+ * $State$
+ * $Revision$
+ */
+
+#include <stdlib.h>
+#include <unistd.h>
+#include <errno.h>
+#include "libsys.h"
+
+#define	OUT_OF_MEMORY (void*)(-1)	/* sbrk returns this on failure */
+
+static char* current = NULL;
+ 
+void* sbrk(intptr_t increment)
+{
+	char* old;
+	char* new;
+	char* actual;
+	
+	if (!current)
+		current = (char*) _syscall(__NR_brk, 0, 0, 0);
+
+	if (increment == 0)
+		return current;
+		
+	old = current;
+	new = old + increment;
+
+	actual = (char*) _syscall(__NR_brk, (quad) new, 0, 0);
+	if (actual < new)
+	{
+		errno = ENOMEM;
+		return OUT_OF_MEMORY;
+	}
+		
+	current = actual;
+	return old;
+}