Support checking gdb.exe on Windows (#1001)

* 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

@@ -541,6 +541,8 @@ private static string ResolveFromPath(string command)
         {
             string pathVar = System.Environment.GetEnvironmentVariable("PATH");
 
+            bool checkForWindowsExe = PlatformUtilities.IsWindows() && String.IsNullOrEmpty(Path.GetExtension(command));
+
             // Check each portion of the PATH environment variable to see if it contains the requested file
             foreach (string pathPart in pathVar.Split(Path.PathSeparator))
             {
@@ -551,6 +553,15 @@ private static string ResolveFromPath(string command)
                 {
                     return candidate;
                 }
+
+                if (checkForWindowsExe)
+                {
+                    string exeCandidate = candidate + ".exe";
+                    if (File.Exists(exeCandidate))
+                    {
+                        return exeCandidate;
+                    }
+                }
             }
 
             return null;