powerpc/boot: Explicitly disable usage of SPE instructions

uImage boot wrapper should not use SPE instructions, like kernel itself.
Boot wrapper has already disabled Altivec and VSX instructions but not SPE.
Options -mno-spe and -mspe=no already set when compilation of kernel, but
not when compiling uImage wrapper yet. Fix it.

Cc: stable@vger.kernel.org
Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220827134454.17365-1-pali@kernel.org