powerpc/Makefile: set -mcpu=860 flag for the 8xx

powerpc/Makefile: set -mcpu=860 flag for the 8xx

When compiled with GCC 8.1, vmlinux is significantly bigger than
with GCC 4.8.

When looking at the generated code with objdump, we notice that
all functions and loops when a 16 bytes alignment. This significantly
increases the size of the kernel. It is pointless and even
counterproductive as on the 8xx 'nop' also consumes one clock cycle.

Size of vmlinux with GCC 4.8:
   text	   data	    bss	    dec	    hex	filename
5801948	1626076	 457796	7885820	 7853fc	vmlinux

Size of vmlinux with GCC 8.1:
   text	   data	    bss	    dec	    hex	filename
6764592	1630652	 456476	8851720	 871108	vmlinux

Size of vmlinux with GCC 8.1 and this patch:
   text	   data	    bss	    dec	    hex	filename
6331544	1631756	 456476	8419776	 8079c0	vmlinux

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>