Link to libgcc when statically linking musl

src/unix/mod.rs

@@ -285,6 +285,16 @@ cfg_if! {
         // cargo build, don't pull in anything extra as the libstd dep
         // already pulls in all libs.
     } else if #[cfg(target_env = "musl")] {
+        // On some architectures (e.g. aarch64) musl depends on some libgcc
+        // functions (__addtf3, __multf3, __subtf3) for long double arithmetic that
+        // it uses internally. Unfortunately we don't provide these functions in
+        // compiler-builtins, so we instead need to get them from libgcc.
+        //
+        // This is not a problem if we are linking to libc dynamically since the
+        // libgcc dependency will automatically get picked up by the linker then.
+        #[cfg_attr(feature = "stdbuild",
+                   link(name = "gcc", kind = "static",
+                        cfg(target_feature = "crt-static")))]
         #[cfg_attr(feature = "stdbuild",
                    link(name = "c", kind = "static",
                         cfg(target_feature = "crt-static")))]