[LLD] Allow configuring default ld.lld backend

The motivation for this is ld.lld --help targeting MinGW which
currently prints help for the ELF backend unless -m i386pe{,p} is
added. This confuses build systems that grep through linker help to
find supported flags.

This matches LD from Binutils which always prints help for MinGW
when configured to target it.

After this change, the backend can still be overridden to any
supported ELF/MinGW target by using correct -m <arch>.

Differential Revision: https://reviews.llvm.org/D87418

lld/CMakeLists.txt

@@ -174,6 +174,12 @@ endif()
 option(LLD_BUILD_TOOLS
   "Build the lld tools. If OFF, just generate build targets." ON)
 
+option(LLD_DEFAULT_LD_LLD_IS_MINGW
+    "Use MinGW as the default backend for ld.lld. If OFF, ELF will be used." OFF)
+if (LLD_DEFAULT_LD_LLD_IS_MINGW)
+  add_definitions("-DLLD_DEFAULT_LD_LLD_IS_MINGW=1")
+endif()
+
 if (MSVC)
   add_definitions(-wd4530) # Suppress 'warning C4530: C++ exception handler used, but unwind semantics are not enabled.'
   add_definitions(-wd4062) # Suppress 'warning C4062: enumerator X in switch of enum Y is not handled' from system header.

lld/tools/lld/lld.cpp

@@ -92,7 +92,12 @@ static bool isPETarget(std::vector<const char *> &v) {
       continue;
     return isPETargetName(*(it + 1));
   }
+
+#ifdef LLD_DEFAULT_LD_LLD_IS_MINGW
+  return true;
+#else
   return false;
+#endif
 }
 
 static Flavor parseProgname(StringRef progname) {