[LLD] [MinGW] Fall back to using default target if no -m flag given.

On Cygwin at least, GCC is not pasing any -m flag to the linker, so fall
back to the default target triple to determine if we need to apply
i386-specific behaviors.

Fixes #134558

Author: jeremyd2019

lld/MinGW/Driver.cpp

@@ -171,6 +171,15 @@ searchLibrary(StringRef name, ArrayRef<StringRef> searchPaths, bool bStatic) {
   return "";
 }
 
+static bool
+isI386Target(const opt::InputArgList &args, const Triple &defaultTarget)
+{
+  auto *a = args.getLastArg(OPT_m);
+  if (a)
+    return StringRef(a->getValue()) == "i386pe";
+  return defaultTarget.getArch() == Triple::x86;
+}
+
 namespace lld {
 namespace coff {
 bool link(ArrayRef<const char *> argsArr, llvm::raw_ostream &stdoutOS,
@@ -216,6 +225,8 @@ bool link(ArrayRef<const char *> argsArr, llvm::raw_ostream &stdoutOS,
     return false;
   }
 
+  Triple defaultTarget(Triple::normalize(sys::getDefaultTargetTriple()));
+
   std::vector<std::string> linkArgs;
   auto add = [&](const Twine &s) { linkArgs.push_back(s.str()); };
 
@@ -224,7 +235,7 @@ bool link(ArrayRef<const char *> argsArr, llvm::raw_ostream &stdoutOS,
 
   if (auto *a = args.getLastArg(OPT_entry)) {
     StringRef s = a->getValue();
-    if (args.getLastArgValue(OPT_m) == "i386pe" && s.starts_with("_"))
+    if (isI386Target(args, defaultTarget) && s.starts_with("_"))
       add("-entry:" + s.substr(1));
     else if (!s.empty())
       add("-entry:" + s);
@@ -521,7 +532,7 @@ bool link(ArrayRef<const char *> argsArr, llvm::raw_ostream &stdoutOS,
   for (auto *a : args.filtered(OPT_Xlink))
     add(a->getValue());
 
-  if (args.getLastArgValue(OPT_m) == "i386pe")
+  if (isI386Target(args, defaultTarget))
     add("-alternatename:__image_base__=___ImageBase");
   else
     add("-alternatename:__image_base__=__ImageBase");