x86/imdct36: don't build imdct36_float_sse on x86_64 targets

There's an SSE2 version as well, and x86_64 guarantees that
instruction set is present.

Signed-off-by: James Almer <jamrial@gmail.com>
Reviewed-by: "Ronald S. Bultje" <rsbultje@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>