Provide libgcc linker script workaround for NDK >= 23

libgcc was removed from NDK v23 which breaks Rust builds against
pre-built standard libraries that were linked against libgcc
instead of libunwind.

As a workaround this ensures that there is a libgcc.a in the
library search paths which is a linker script that will redirect
to link with libunwind instead.

The libgcc.a linker script is created under:

 target/cargo-ndk/libgcc_workaround/libgcc.a

Fixes: bbqsrc#22

src/cargo.rs

@@ -1,7 +1,9 @@
+use std::io::{Write, Result};
 use std::path::{Path, PathBuf};
 use std::process::Command;
 
 use cargo_metadata::Version;
+use cargo_metadata::camino::Utf8PathBuf;
 
 #[cfg(target_os = "macos")]
 const ARCH: &str = "darwin-x86_64";
@@ -75,8 +77,19 @@ fn cargo_env_target_cfg(triple: &str, key: &str) -> String {
     format!("CARGO_TARGET_{}_{}", &triple.replace("-", "_"), key).to_uppercase()
 }
 
+fn create_libgcc_linker_script_workaround(target_dir: &Utf8PathBuf) -> Result<Utf8PathBuf> {
+    let libgcc_workaround_dir = target_dir.join("cargo-ndk").join("libgcc_workaround");
+    std::fs::create_dir_all(&libgcc_workaround_dir)?;
+    let libgcc_workaround_file = libgcc_workaround_dir.join("libgcc.a");
+    let mut file = std::fs::File::create(libgcc_workaround_file)?;
+    file.write_all(b"INPUT(-lunwind)")?;
+
+    Ok(libgcc_workaround_dir)
+}
+
 pub(crate) fn run(
     dir: &Path,
+    target_dir: &Utf8PathBuf,
     ndk_home: &Path,
     version: Version,
     triple: &str,
@@ -133,6 +146,22 @@ pub(crate) fn run(
         .env(cargo_env_target_cfg(triple, "linker"), &target_linker)
         .args(cargo_args);
 
+    // NDK releases >= 23 no longer include libgcc which rust's pre-built standard libraries
+    // depend on. As a workaround for newer NDKs we redirect libgcc to libunwind.
+    // Ref: https://github.com/rust-lang/rust/pull/85806
+    if version.major >= 23 {
+        match create_libgcc_linker_script_workaround(target_dir) {
+            Ok(libdir) => {
+                cargo_cmd.env("RUSTFLAGS", format!("-L{libdir}"));
+            }
+            Err(e) => {
+                log::error!("Failed to create libgcc.a linker script workaround");
+                log::error!("{}", e);
+                std::process::exit(1);
+            }
+        }
+    }
+
     let extra_include = format!("{}/usr/include/{}", &target_sysroot.display(), triple);
     if bindgen {
         let bindgen_args = format!(

src/cli.rs

@@ -272,6 +272,7 @@ pub(crate) fn run(args: Vec<String>) {
 
         let status = crate::cargo::run(
             &working_dir,
+            &metadata.target_directory,
             &ndk_home,
             ndk_version.clone(),
             triple,