gdb: More cleanup changes

- Move the message handler into its debugger class part,
- Move all message types into one file and collapse 3 of the ones with no data into a generic, stateless message with a single property being its type,
- Add an Fpscr helper property on IExecutionContext along with a comment about what Fpscr is (similar to the other registers in there)
- Moved the Rcmd helpers (such as GetRegisters, GetMinidump, etc) into a dedicated Debugger class part,
- Fixed the double-collection (ToArray being called twice) in GetThreadUids & GetThread in KProcess

src/Ryujinx.HLE/Debugger/Debugger.MainThread.cs

@@ -64,10 +64,10 @@ namespace Ryujinx.HLE.Debugger
                                 Logger.Notice.Print(LogClass.GdbStub, "NACK received!");
                                 continue;
                             case '\x03':
-                                _messages.Add(new BreakInMessage());
+                                _messages.Add(StatelessMessage.BreakIn);
                                 break;
                             case '$':
-                                string cmd = "";
+                                string cmd = string.Empty;
                                 while (true)
                                 {
                                     int x = _readStream.ReadByte();
@@ -85,7 +85,7 @@ namespace Ryujinx.HLE.Debugger
                                 }
                                 else
                                 {
-                                    _messages.Add(new SendNackMessage());
+                                    _messages.Add(StatelessMessage.SendNack);
                                 }
 
                                 break;