Fix RTCD on ARM with Neon Intrinsics but not ASM.

Notably, ARM floating-point build.

This maintains the invariant that we don't use later instruction
 sets if the OS claims an earlier one is not available.
However, it does not update configure to ensure that there is
 build support for all earlier instruction sets if NEON build
 support is enabled (though I am not aware of a build toolchain
 where this is actually a problem).

Signed-off-by: Timothy B. Terriberry <tterribe@xiph.org>