powerpc: Fix compile error with binutils 2.15

My previous patch to fix compilation with binutils-2.17 causes
a "file truncated" build error from ld with binutils 2.15 (and
possibly older), and a warning with 2.16 and 2.17.

This fixes it.
Signed-off-by: Segher Boessenkool <segher@kernel.crashing.org>
Acked-by: Chuck Meade <chuckmeade@mindspring.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/vmlinux.lds.S

@@ -85,7 +85,7 @@ SECTIONS
 
 	/* The dummy segment contents for the bug workaround mentioned above
 	   near PHDRS.  */
-	.dummy : {
+	.dummy : AT(ADDR(.dummy) - LOAD_OFFSET) {
 		LONG(0xf177)
 	} :kernel :dummy