Use DWARF4 on Windows.

Author: vadimcn

src/librustc/middle/trans/debuginfo.rs

@@ -739,7 +739,7 @@ pub fn finalize(cx: &CrateContext) {
             cx.sess().targ_cfg.os == abi::OsiOS {
             "Dwarf Version".with_c_str(
                 |s| llvm::LLVMRustAddModuleFlag(cx.llmod(), s, 2));
-        } else {
+        } else if cx.sess().targ_cfg.os == abi::OsLinux {
             // 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.