Revert "Linux: Don't default to ld.gold linker"

This reverts commit e125411.

It's too much of a breaking change for a point release.

Author: kinke

driver/linker-gcc.cpp

@@ -587,12 +587,22 @@ void ArgsBuilder::build(llvm::StringRef outputPath,
 void ArgsBuilder::addLinker() {
   llvm::StringRef linker = opts::linker;
 
-  // Default to ld.bfd for Android (placing .tdata and .tbss sections adjacent
-  // to each other as required by druntime's rt.sections_android, contrary to
-  // gold and lld as of Android NDK r21d).
-  if (global.params.targetTriple->getEnvironment() == llvm::Triple::Android &&
+  // We have a default linker preference for Linux targets. It can be disabled
+  // via `-linker=` (explicitly empty).
+  if (global.params.targetTriple->isOSLinux() &&
       opts::linker.getNumOccurrences() == 0) {
-    linker = "bfd";
+    // Default to ld.bfd for Android (placing .tdata and .tbss sections adjacent
+    // to each other as required by druntime's rt.sections_android, contrary to
+    // gold and lld as of Android NDK r21d).
+    if (global.params.targetTriple->getEnvironment() == llvm::Triple::Android) {
+      linker = "bfd";
+    }
+    // Otherwise default to ld.gold for Linux due to ld.bfd issues with ThinLTO
+    // (see #2278) and older bfd versions stripping llvm.used symbols (e.g.,
+    // ModuleInfo refs) with --gc-sections (see #2870).
+    else {
+      linker = "gold";
+    }
   }
 
   if (!linker.empty())