Fix CYCLES_PER_MICROSECOND conflict on STM32F1

Marlin/src/core/macros.h

 #define _O3          __attribute__((optimize("O3")))
 
 // Clock speed factors
-#ifndef CYCLES_PER_MICROSECOND
+#if !defined(CYCLES_PER_MICROSECOND) && !defined(__STM32F1__)
   #define CYCLES_PER_MICROSECOND (F_CPU / 1000000L) // 16 or 20 on AVR
 #endif
 

Marlin/src/pins/pins_STM32F1R.h

  *
  */
 
-#if !defined(__STM32F1__)
+#ifndef __STM32F1__
     #error "Oops!  Make sure you have an STM32F1 board selected from the 'Tools -> Boards' menu."
 #endif