[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: NKumar Gala <galak@kernel.crashing.org>
Signed-off-by: NPaul Mackerras <paulus@samba.org>

arch/powerpc/kernel/setup_32.c

@@ -289,7 +289,7 @@ void __init setup_arch(char **cmdline_p)
 	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;

arch/powerpc/kernel/setup_64.c

@@ -510,7 +510,7 @@ void __init setup_arch(char **cmdline_p)
 	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;

arch/powerpc/kernel/vmlinux.lds.S

 #ifdef CONFIG_PPC64
-#include <asm/page.h>
 #define PROVIDE32(x)	PROVIDE(__unused__##x)
 #else
-#define PAGE_SIZE	4096
-#define KERNELBASE	CONFIG_KERNEL_START
 #define PROVIDE32(x)	PROVIDE(x)
 #endif
+#include <asm/page.h>
 #include <asm-generic/vmlinux.lds.h>
 #include <asm/cache.h>