[POWERPC] Clean up some linker and symbol usage

* PAGE_OFFSET is not always the start of code, use _stext instead.
* grab PAGE_SIZE and KERNELBASE from asm/page.h like ppc64 does.  Makes the
  code a bit more common and provide a single place to manipulate the
  defines for things like kdump.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>
diff --git a/arch/powerpc/kernel/setup_64.c b/arch/powerpc/kernel/setup_64.c
index 2c2d8315..0205d40 100644
--- a/arch/powerpc/kernel/setup_64.c
+++ b/arch/powerpc/kernel/setup_64.c
@@ -510,7 +510,7 @@
 	if (ppc_md.panic)
 		setup_panic();
 
-	init_mm.start_code = PAGE_OFFSET;
+	init_mm.start_code = (unsigned long)_stext;
 	init_mm.end_code = (unsigned long) _etext;
 	init_mm.end_data = (unsigned long) _edata;
 	init_mm.brk = klimit;