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.Cpu/IExecutionContext.cs

@@ -42,6 +42,11 @@ namespace Ryujinx.Cpu
         /// </summary>
         uint Fpsr { get; set; }
 
+        /// <summary>
+        /// Floating-point Status and Control Register.
+        /// </summary>
+        uint Fpscr => Fpsr | Fpcr;
+
         /// <summary>
         /// Indicates whenever the CPU is running 64-bit (AArch64 mode) or 32-bit (AArch32 mode) code.
         /// </summary>