debuginfo: Clamp the maximum dwarf version to 3

This is a consequence of rust-lang#13611 and our bots running a "fairly old" gdb which
doesn't understand the newer versions of dwarf.

src/librustc/middle/trans/debuginfo.rs

@@ -279,6 +279,12 @@ pub fn finalize(cx: &CrateContext) {
         if cx.sess().targ_cfg.os == abi::OsMacos {
             "Dwarf Version".with_c_str(
                 |s| llvm::LLVMRustAddModuleFlag(cx.llmod, s, 2));
+        } else {
+            // FIXME(#13611) this is a kludge fix because the linux bots have
+            //               gdb 7.4 which doesn't understand dwarf4, we should
+            //               do something more graceful here.
+            "Dwarf Version".with_c_str(
+                |s| llvm::LLVMRustAddModuleFlag(cx.llmod, s, 3));
         }
 
         // Prevent bitcode readers from deleting the debug info.