Support checking gdb.exe on Windows

This PR adds additional checks to look for gdb on Windows.
If a user does not specify miDebuggerPath, we will check for gdb and
gdb.exe in PATH. If a miDebuggerPath is provided and does not supply any
directory, we will search that binary and binary.exe in PATH.

src/MICore/LaunchOptions.cs

@@ -551,6 +551,13 @@ private static string ResolveFromPath(string command)
                 {
                     return candidate;
                 }
+
+                string exeCandidate = candidate + ".exe";
+
+                if (PlatformUtilities.IsWindows() && File.Exists(exeCandidate))
+                {
+                    return exeCandidate;
+                }
             }
 
             return null;