Minor string storage optimization

Marlin/src/MarlinCore.cpp

            M21_STR[] PROGMEM = "M21",
            M23_STR[] PROGMEM = "M23 %s",
            M24_STR[] PROGMEM = "M24",
+           SP_P_STR[] PROGMEM = " P",
+           SP_T_STR[] PROGMEM = " T",
            SP_X_STR[] PROGMEM = " X",
            SP_Y_STR[] PROGMEM = " Y",
            SP_Z_STR[] PROGMEM = " Z",

Marlin/src/MarlinCore.h

 #endif
 
 extern const char NUL_STR[], M112_KILL_STR[], G28_STR[], M21_STR[], M23_STR[], M24_STR[],
-                  SP_X_STR[], SP_Y_STR[], SP_Z_STR[], SP_E_STR[];
+                  SP_P_STR[], SP_T_STR[], SP_X_STR[], SP_Y_STR[], SP_Z_STR[], SP_E_STR[];

Marlin/src/gcode/config/M200-M205.cpp

   if (!parser.seen("PRST")) {
     SERIAL_ECHOPAIR("Acceleration: P", planner.settings.acceleration);
     SERIAL_ECHOPAIR(" R", planner.settings.retract_acceleration);
-    SERIAL_ECHOLNPAIR(" T", planner.settings.travel_acceleration);
+    SERIAL_ECHOLNPAIR_P(SP_T_STR, planner.settings.travel_acceleration);
   }
   else {
     //planner.synchronize();

Marlin/src/gcode/config/M217.cpp

     serialprintPGM(eeprom ? PSTR("  M217") : PSTR("Toolchange:"));
     SERIAL_ECHOPAIR(" S", LINEAR_UNIT(toolchange_settings.swap_length));
     SERIAL_ECHOPAIR_P(SP_E_STR, LINEAR_UNIT(toolchange_settings.extra_prime));
-    SERIAL_ECHOPAIR(" P", LINEAR_UNIT(toolchange_settings.prime_speed));
+    SERIAL_ECHOPAIR_P(SP_P_STR, LINEAR_UNIT(toolchange_settings.prime_speed));
     SERIAL_ECHOPAIR(" R", LINEAR_UNIT(toolchange_settings.retract_speed));
 
     #if ENABLED(TOOLCHANGE_PARK)

Marlin/src/gcode/control/M605.cpp

         DEBUG_EOL();
 
         HOTEND_LOOP() {
-          DEBUG_ECHOPAIR(" T", int(e));
+          DEBUG_ECHOPAIR_P(SP_T_STR, int(e));
           LOOP_XYZ(a) DEBUG_ECHOPAIR("  hotend_offset[", int(e), "].", axis_codes[a] | 0x20, "=", hotend_offset[e][a]);
           DEBUG_EOL();
         }

Marlin/src/gcode/feature/mixing/M166.cpp

 
 inline void echo_zt(const int t, const float &z) {
   mixer.update_mix_from_vtool(t);
-  SERIAL_ECHOPAIR_P(SP_Z_STR, z, PSTR(" T"), t);
+  SERIAL_ECHOPAIR_P(SP_Z_STR, z, SP_T_STR, t);
   echo_mix();
 }
 

Marlin/src/gcode/queue.cpp

       while (NUMERIC_SIGNED(*p))
         SERIAL_ECHO(*p++);
     }
-    SERIAL_ECHOPGM(" P"); SERIAL_ECHO(int(BLOCK_BUFFER_SIZE - planner.movesplanned() - 1));
-    SERIAL_ECHOPGM(" B"); SERIAL_ECHO(BUFSIZE - length);
+    SERIAL_ECHOPAIR_P(SP_P_STR, int(BLOCK_BUFFER_SIZE - planner.movesplanned() - 1));
+    SERIAL_ECHOPAIR(" B", BUFSIZE - length);
   #endif
   SERIAL_EOL();
 }

Marlin/src/module/configuration_store.cpp

 
     CONFIG_ECHO_HEADING("Acceleration (units/s2): P<print_accel> R<retract_accel> T<travel_accel>");
     CONFIG_ECHO_START();
-    SERIAL_ECHOLNPAIR(
-        "  M204 P", LINEAR_UNIT(planner.settings.acceleration)
-      , " R", LINEAR_UNIT(planner.settings.retract_acceleration)
-      , " T", LINEAR_UNIT(planner.settings.travel_acceleration)
+    SERIAL_ECHOLNPAIR_P(
+        PSTR("  M204 P"), LINEAR_UNIT(planner.settings.acceleration)
+      , PSTR(" R"), LINEAR_UNIT(planner.settings.retract_acceleration)
+      , SP_T_STR, LINEAR_UNIT(planner.settings.travel_acceleration)
     );
 
     if (!forReplay) {
     SERIAL_ECHOLNPAIR_P(
         PSTR("  M205 B"), LINEAR_UNIT(planner.settings.min_segment_time_us)
       , PSTR(" S"), LINEAR_UNIT(planner.settings.min_feedrate_mm_s)
-      , PSTR(" T"), LINEAR_UNIT(planner.settings.min_travel_feedrate_mm_s)
+      , SP_T_STR, LINEAR_UNIT(planner.settings.min_travel_feedrate_mm_s)
       #if DISABLED(CLASSIC_JERK)
         , PSTR(" J"), LINEAR_UNIT(planner.junction_deviation_mm)
       #endif
       CONFIG_ECHO_START();
       SERIAL_ECHOLNPAIR_P(
           PSTR("  M665 S"), delta_segments_per_second
-        , PSTR(" P"), scara_home_offset.a
-        , PSTR(" T"), scara_home_offset.b
+        , SP_P_STR, scara_home_offset.a
+        , SP_T_STR, scara_home_offset.b
         , SP_Z_STR, LINEAR_UNIT(scara_home_offset.z)
       );
 
           SERIAL_ECHOPAIR_P(
             #if HOTENDS > 1 && ENABLED(PID_PARAMS_PER_HOTEND)
               PSTR("  M301 E"), e,
-              PSTR(" P")
+              SP_P_STR
             #else
               PSTR("  M301 P")
             #endif

Marlin/src/module/temperature.cpp

     const user_thermistor_t &t = user_thermistor[t_index];
 
     SERIAL_ECHOPAIR_F(" R", t.series_res, 1);
-    SERIAL_ECHOPAIR_F(" T", t.res_25, 1);
+    SERIAL_ECHOPAIR_F_P(SP_T_STR, t.res_25, 1);
     SERIAL_ECHOPAIR_F(" B", t.beta, 1);
     SERIAL_ECHOPAIR_F(" C", t.sh_c_coeff, 9);
     SERIAL_ECHOPGM(" ; ");