powerpc/boot: Specify ABI v2 when building an LE boot wrapper

The kernel does it, not the boot wrapper, which breaks with some
cross compilers that still default to ABI v1.

Fixes: 147c0516 ("powerpc/boot: Add support for 64bit little endian wrapper")
Cc: stable@vger.kernel.org # v3.16+
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>