cherry-pick to 0.16.20, which is what rustls currently requires

now to layer in xous build but relying on ec_17 libs

this is technically a generated file but it's very helpful
to view its source to understand how things work

the unused_assignments lint is a bit more complicated to fix up
because there are two cases where this tends to happen:

1. c2rust will faithfully assign a return value to a dummy variable
if that's what the C code actually did.

2. c2rust will always assign an initial value to a variable even
if it is never used before actually being overwritten in the code

I think we have the parsing infrastrucutre we'd need to remove that
lint but i think we'll allow it for now...

the remaining we'll let stand because the snake case and
camel case conventions of crypto code does not obey Rust
conventions for good reasons.

Change macro to match non-x86 arches.

Update chacha.rs

…nto 0.16.20-cleanup