Debugger: Optimize cpu_thread::dump_regs()

Reuse string buffer. Copies and reallocations are expensive with such large strings.

rpcs3/rpcs3qt/debugger_frame.h

@@ -57,6 +57,7 @@ class debugger_frame : public custom_dock_widget
 	system_state m_emu_state{};
 	u32 m_last_pc = -1;
 	std::vector<char> m_last_query_state;
+	std::string m_last_reg_state;
 	u32 m_last_step_over_breakpoint = -1;
 	u64 m_ui_update_ctr = 0;
 	u64 m_ui_fast_update_permission_deadline = 0;