powerpc: Make sure _etext is after all kernel text

This makes core_kernel_text() (and therefore kernel_text_address())
return the correct result.  Currently all the __devinit routines (at
least) will not be considered to be kernel text.

This is just a quick fix for 2.6.27 - hopefully we will be able to fix
this better in 2.6.28.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>