[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>