[dev.power64] cmd/ld: add MAXIO back in lib.h

It was accidentally removed.

LGTM=dave
R=rsc, iant, dave
CC=golang-codereviews
https://golang.org/cl/124880043

src/cmd/ld/lib.h

@@ -33,6 +33,10 @@
 // A section further describes the pieces of that block for
 // use in debuggers and such.
 
+enum {
+	MAXIO		= 8192,
+};
+
 typedef struct Segment Segment;
 typedef struct Section Section;