Support Android ndk versions r23-beta3 and up

Since android ndk version `r23-beta3`, `libgcc` has been replaced with
`libunwind`. This moves the linking of `libgcc`/`libunwind` into the
`unwind` crate where we check if the system compiler can find
`libunwind` and fall back to `libgcc` if needed.

Author: Tilmann Meyer

library/std/src/sys/unix/mod.rs

@@ -210,7 +210,6 @@ cfg_if::cfg_if! {
     if #[cfg(target_os = "android")] {
         #[link(name = "dl")]
         #[link(name = "log")]
-        #[link(name = "gcc")]
         extern "C" {}
     } else if #[cfg(target_os = "freebsd")] {
         #[link(name = "execinfo")]

library/unwind/build.rs

@@ -20,6 +20,20 @@ fn main() {
         // linking for Linux is handled in lib.rs
         if target.contains("musl") {
             llvm_libunwind::compile();
+        } else if target.contains("android") {
+            let build = cc::Build::new();
+
+            // Since ndk r23 beta 3 `libgcc` was replaced with `libunwind` thus
+            // check if we have `libunwind` available and if so use it. Otherwise
+            // fall back to `libgcc` to support older ndk versions.
+            let has_unwind =
+                build.is_flag_supported("-lunwind").expect("Unable to invoke compiler");
+
+            if has_unwind {
+                println!("cargo:rustc-link-lib=unwind");
+            } else {
+                println!("cargo:rustc-link-lib=gcc");
+            }
         }
     } else if target.contains("freebsd") {
         println!("cargo:rustc-link-lib=gcc_s");