powerpc/boot: Fix boot wrapper code generation with CONFIG_POWER10_CPU

-mcpu=power10 will generate prefixed and pcrel code by default, which
we do not support. The general kernel disables these with cflags, but
those were missed for the boot wrapper.

Fixes: 4b2a9315

 ("powerpc/64s: POWER10 CPU Kconfig build option")
Cc: stable@vger.kernel.org # v6.1+
Reported-by: Danny Tsen <dtsen@linux.ibm.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230407040909.230998-1-npiggin@gmail.com