powerpc/64: Do not link crtsaveres.o in boot

crtsaveres.S is empty with 64-bit builds already, so just don't
build and link it to match the vmlinux build.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
[mpe: Use CONFIG_PPC64_BOOT_WRAPPER not CONFIG_PPC32 to fix BE build]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>