This fixes "unnecessary `unsafe` block" warnings encountered when
building the generated rust_programs.
The only pattern that actually required `unsafe` was transmuting bit
patterns into floats. This patch uses the safe `from_bits` instead, but
because that isn't const, we have to make them local let-bound
variables.
This involves moving from the ACLE intrinsic definitions (which aren't
available for SVE at this point) to a JSON file. This was derived from
ARM's documentation[^1], and then relicensed under `MIT OR Apache-2.0` for
use in this repository.
[^1]: https://developer.arm.com/architectures/instruction-sets/intrinsics
