unwind: Move linux-gnu library linking to lib.rs and libc

This unifies it with the handling of `target-feature=+crt-static` on
other platforms, and allows for supporting static glibc in the future.

library/unwind/build.rs

@@ -12,11 +12,9 @@ fn main() {
     } else if target.contains("x86_64-fortanix-unknown-sgx") {
         llvm_libunwind::compile();
     } else if target.contains("linux") {
+        // linking for Linux is handled in lib.rs
         if target.contains("musl") {
-            // linking for musl is handled in lib.rs
             llvm_libunwind::compile();
-        } else if !target.contains("android") {
-            println!("cargo:rustc-link-lib=gcc_s");
         }
     } else if target.contains("freebsd") {
         println!("cargo:rustc-link-lib=gcc_s");

library/unwind/src/lib.rs

@@ -42,6 +42,10 @@ cfg_if::cfg_if! {
 #[link(name = "gcc_s", cfg(not(target_feature = "crt-static")))]
 extern "C" {}
 
+#[cfg(all(target_os = "linux", target_env = "gnu", not(feature = "llvm-libunwind")))]
+#[link(name = "gcc_s", cfg(not(target_feature = "crt-static")))]
+extern "C" {}
+
 #[cfg(target_os = "redox")]
 #[link(name = "gcc_eh", kind = "static-nobundle", cfg(target_feature = "crt-static"))]
 #[link(name = "gcc_s", cfg(not(target_feature = "crt-static")))]