[POWERPC] Specify GNUTARGET on $(AR) invocations

Without this, some versions of GNU ar fail to create
an archive index if the object files it is packing
together are of a different object format than ar's
default format (for example, binutils compiled to
default to 64-bit, with 32-bit objects).

Signed-off-by: Segher Boessenkool <segher@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>