Auto merge of #1034 - Amanieu:musl_libgcc, r=alexcrichton

Link to libgcc when statically linking 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.

Fixes:
rust-lang/rust#46651
rust-lang/compiler-builtins#201
rust-lang/compiler-builtins#217

Author: bors

src/unix/mod.rs

@@ -285,6 +285,18 @@ 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")))]