Description:

AWS-LC can now optionally generate and install Rust bindings at $PREFIX/share/rust/aws_lc_bindings.rs. This PR teaches openssl-sys to discover and use those bindings, removing the need for bindgen (or bindgen-cli) at build time when they're present.

The detection logic lives in try_pregenerated_awslc_bindings, which looks for the file relative to the include directory. When found, it copies the bindings into OUT_DIR/bindgen.rs and short-circuits both bindgen and the static wrapper C compilation step. If the file isn't present, the existing bindgen / bindgen-cli path runs exactly as before. Both the bindgen feature and non-bindgen code paths are covered.

Because the pregenerated bindings don't include static inline functions or C preprocessor macros, pure-Rust implementations of ERR_GET_LIB, ERR_GET_REASON, ERR_GET_FUNC, and BIO_get_mem_data are provided in the aws_lc module. The existing BIO_get_mem_data shim was already there; the error helpers are new and match the logic in aws-lc-sys. These shims are gated with #[cfg(any(feature = "aws-lc", feature = "aws-lc-fips", awslc_pregenerated))] to avoid conflicting with the extern "C" declarations that bindgen's wrap_static_fns produces on the normal (non-pregenerated) path.

A few things worth noting for review:

• The pregenerated bindings use ::std::os::raw::* types rather than ::libc::* (since they weren't generated with ctypes_prefix). These are the same underlying types, so there's no compatibility issue.
• #[allow(unpredictable_function_pointer_comparisons)] is added to the aws_lc module because the pregenerated bindings derive PartialEq on structs containing function pointer fields. When using aws-lc-sys as a crate dependency this lint is scoped to that crate; with include!-d bindings it surfaces here instead.
• All three code paths have been tested: pregenerated bindings via OPENSSL_DIR, normal bindgen via OPENSSL_DIR (without pregenerated bindings), and the aws-lc crate feature.