powerpc: Fix vmlinux.lds.S for 32-bit

We can't currently use asm-ppc/page.h in vmlinux.lds.S, so until
we have a merged page.h, define PAGE_SIZE and KERNELBASE locally.
Also gets rid of some dynamic executable cruft that we had for
32-bit.  With -Ttext=$(KERNELBASE) this didn't cause any problem,
but when we changed to putting . = KERNELBASE in the vmlinux.lds.S
this cruft caused the text to get linked at 0xa0 instead of
0xc0000000.  Oops.

Signed-off-by: Paul Mackerras <paulus@samba.org>