target/ppc: fix build on ppc64 host

When I try to build a ppc64 target on a ppc64 host (gcc 8.1.1), I have:

.../target/ppc/int_helper.c: In function 'helper_vinsertb':
.../target/ppc/int_helper.c:1954:32: error: array subscript 18446744073709551608 is above array bounds of 'uint8_t[16]' {aka 'unsigned char[16]'} [-Werror=array-bounds]
         memmove(&r->u8[index], &b->u8[8 - sizeof(r->element)],              \
                                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.../target/ppc/int_helper.c:1965:1: note: in expansion of macro 'VINSERT'

If we compare with the macro for ppc64le, we can see
sizeof(r->element[0]) should be used instead of sizeof(r->element).

And VINSERT uses only u8, u16, u32 and u64, so the maximum value
of sizeof(r->element[0]) is 8

Suggested-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>