set subsections_via_symbols for ld64 helper sections

Author: usamoi

Diff for: compiler/rustc_codegen_ssa/src/back/metadata.rs

@@ -221,6 +221,9 @@ pub(crate) fn create_object_file(sess: &Session) -> Option<write::Object<'static
 
         file.set_macho_build_version(macho_object_build_version_for_target(sess))
     }
+    if binary_format == BinaryFormat::MachO {
+        file.set_subsections_via_symbols();
+    }
     if binary_format == BinaryFormat::Coff {
         // Disable the default mangler to avoid mangling the special "@feat.00" symbol name.
         let original_mangling = file.mangling();

Diff for: tests/run-make/executable-no-mangle-strip/main.rs

@@ -0,0 +1,16 @@
+unsafe extern "C" {
+    unsafe static UNDEFINED: usize;
+}
+
+#[unsafe(no_mangle)]
+pub fn used() {
+    println!("UNDEFINED = {}", unsafe { UNDEFINED });
+}
+
+pub fn marker() -> usize {
+    0
+}
+
+fn main() {
+    assert_eq!(marker(), 0_usize);
+}

Diff for: tests/run-make/executable-no-mangle-strip/rmake.rs

@@ -0,0 +1,9 @@
+// Test that functions marked with `#[no_mangle]` are GC-ed
+
+//@ only-apple
+
+use run_make_support::rustc;
+
+fn main() {
+    rustc().arg("main.rs").run();
+}