simple_idct.asm is 32 bit-only since
bfb28b5ce8,
whereas simple_idct10.asm is x64-only. So don't build
the ultimately unneeded and empty files, as some linkers
complain about this: "ranlib: file:
libavcodec/libavcodec.a(simple_idct.o) has no symbols"
(this is from an Xcode toolchain as reported by Ronald S. Bultje).
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>