powerpc/vmlinux.lds: Move _edata down

Currently _edata does not include several data sections, this causes
the kernel's report of memory usage at boot to not match reality, and
also prevents kmemleak from working - because it scan between _sdata
and _edata for pointers to allocated memory.

This mirrors a similar change made recently to the x86 linker script.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>