Apply some #ifndef

Marlin/src/HAL/HAL_STM32/fastio_STM32.h

@@ -45,7 +45,7 @@ void FastIO_init(); // Must be called before using fast io macros
 
 #define _BV32(b) (1UL << (b))
 
-#if !defined(PWM)
+#ifndef PWM
   #define PWM OUTPUT
 #endif
 

Marlin/src/feature/mixing.h

@@ -25,7 +25,7 @@
 
 //#define MIXER_NORMALIZER_DEBUG
 
-#if !defined(__AVR__) // || DUAL_MIXING_EXTRUDER
+#ifndef __AVR__ // || DUAL_MIXING_EXTRUDER
   // Use 16-bit (or fastest) data for the integer mix factors
   typedef uint_fast16_t mixer_comp_t;
   typedef uint_fast16_t mixer_accu_t;

Marlin/src/pins/stm32/pins_THE_BORG.h

@@ -21,7 +21,7 @@
  */
 #pragma once
 
-#if !defined(STM32F7)
+#ifndef STM32F7
   #error "Oops! Select an STM32F7 board in 'Tools > Board.'"
 #elif HOTENDS > 3 || E_STEPPERS > 3
   #error "The-Borg supports up to 3 hotends / E-steppers."