Improve error message for mingw64 architecture

src/tools/EpicsHostArch.pl

@@ -52,7 +52,13 @@ sub HostArch {
             }
             die "$0: macOS CPU type '$cpu' not recognized\n";
         }
-
+        # mingw64 has 32bit and 64bit build shells which give same arch result
+        if (m/^x86_64-msys/ or m/^i686-msys/) {
+            die "$0: Architecture '$arch' ambiguous.\n".
+                "Rerun after typing one of the following:\n".
+                "    export EPICS_HOST_ARCH=windows-x64-mingw   # for 64bit build\n".
+                "    export EPICS_HOST_ARCH=win32-x86-mingw     # for 32bit build\n";
+        }
         die "$0: Architecture '$arch' not recognized\n";
     }
 }