Exclude non-BoringSSL items from bindings

[kornelski]

Rust started warning about incompatible redefinitions of malloc.

Boring headers pull in C stdlib, and bindgen isn't very good at filtering out its unused/irrelevant symbols.

This change excludes about 3000 lines of boilerplate from the bindings. I couldn't simply exclude __.*, because macOS uses a pattern of time_t = __darwin_time_t, and bindgen would naively leave it broken instead of following the aliases.

[Lekensteyn]

Some more failures on Android due to signal-related symbols, https://github.com/cloudflare/boring/actions/runs/31857179845/job/94944070421?pr=520

error[E0425]: cannot find type `__sighandler_t` in this scope
    --> /home/runner/work/boring/boring/.cache/build-dir/x86_64-linux-android/debug/build/boring-sys-95dbbaded2f700d3/out/bindings.rs:6719:21
     |
6719 |     pub sa_handler: __sighandler_t,
     |                     ^^^^^^^^^^^^^^ not found in this scope

error[E0425]: cannot find type `__sigrestore_t` in this scope
    --> /home/runner/work/boring/boring/.cache/build-dir/x86_64-linux-android/debug/build/boring-sys-95dbbaded2f700d3/out/bindings.rs:6721:22
     |
6721 |     pub sa_restorer: __sigrestore_t,
     |                      ^^^^^^^^^^^^^^ not found in this scope

error[E0425]: cannot find type `sigset_t` in this scope
    --> /home/runner/work/boring/boring/.cache/build-dir/x86_64-linux-android/debug/build/boring-sys-95dbbaded2f700d3/out/bindings.rs:6722:18
     |
6722 |     pub sa_mask: sigset_t,
     |                  ^^^^^^^^ not found in this scope

error[E0425]: cannot find type `sigval_t` in this scope
    --> /home/runner/work/boring/boring/.cache/build-dir/x86_64-linux-android/debug/build/boring-sys-95dbbaded2f700d3/out/bindings.rs:6827:18
     |
6775 | pub union sigval {
     | ---------------- similarly named union `sigval` defined here
...
6827 |     pub _sigval: sigval_t,
     |                  ^^^^^^^^
     |

Aside from that there are also warnings, but these exist before this PR:

[boring-sys 5.2.0] android toolchain=/usr/local/lib/android/sdk/ndk/27.3.13750724/build/cmake/android.toolchain.cmake
[boring-sys 5.2.0] CMAKE_GENERATOR = None
[boring-sys 5.2.0] cargo:warning=Compiler family detection failed due to error: ToolNotFound: failed to find tool "x86_64-linux-android-clang": No such file or directory (os error 2)
[boring-sys 5.2.0] cargo:warning=Compiler family detection failed due to error: ToolNotFound: failed to find tool "x86_64-linux-android-clang++": No such file or directory (os error 2)
[boring-sys 5.2.0] cargo:warning=Compiler family detection failed due to error: ToolNotFound: failed to find tool "x86_64-linux-android-clang": No such file or directory (os error 2)
[boring-sys 5.2.0] CMAKE_PREFIX_PATH_x86_64-linux-android = None
[boring-sys 5.2.0] CMAKE_PREFIX_PATH_x86_64_linux_android = None

[Lekensteyn]

#529 has an alternative approach to exclude non-BoringSSL symbols that should be more robust.

Are the "ptr casts" and Clippy changes still needed? I don't see any warnings in a recent run, and neither have these commits been applied to current master yet.