powerpc: Move bad_stack() below the fwnmi_data_area

At the moment the allmodconfig build is failing because we run out of
space between altivec_assist() at 0x5700 and the fwnmi_data_area at
0x7000.

Fixing it permanently will take some more work, but a quick fix is to
move bad_stack() below the fwnmi_data_area. That gives us just enough
room with everything enabled.

bad_stack() is called from the common exception handlers, but it's a
non-conditional branch, so we have plenty of scope to move it further
way.

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