warning: extern block uses type [u64; 4], which is not FFI-safe

[egrimley-arm]

In the output from the CI tests there are about ten warnings similar to this one:

   Compiling psa-crypto-sys v0.10.0 (/home/runner/work/rust-psa-crypto/rust-psa-crypto/psa-crypto-sys)
warning: `extern` block uses type `[u64; 4]`, which is not FFI-safe
 --> /home/runner/work/rust-psa-crypto/rust-psa-crypto/target/aarch64-unknown-linux-gnu/debug/build/psa-crypto-sys-0bf6a434edbfaaab/out/shim_bindings.rs:3:154378
  |
3 | ...at : * const :: std :: os :: raw :: c_char , __arg : [u64 ; 4usize]) -> :: std :: os :: raw :: c_int ; } extern "C" { pub fn vprintf (...
  |                                                         ^^^^^^^^^^^^^^ not FFI-safe
  |
  = note: `#[warn(improper_ctypes)]` on by default
  = help: consider passing a pointer to the array
  = note: passing raw arrays by value is not FFI-safe

The warnings happen when building for AArch64 and apparently relate to va_list. They are probably harmless so we don't have to do anything about it, but maybe we'll want to turn warnings into errors one day. Possible actions:

1. Do nothing.
2. If the functions are not used, tell bindgen not to generate bindings for those functions: is there a better way of doing that than listing all the functions somewhere in build.rs?
3. Disable that particular warning for shim_bindings.rs.