Make F_CPU a compile-time constant (#21051)

Marlin/src/HAL/STM32/HAL.cpp

 void HAL_init() {
   FastIO_init();
 
+  // Ensure F_CPU is a constant expression. 
+  // If the compiler breaks here, it means that delay code that should compute at compile time will not work.
+  // So better safe than sorry here.
+  constexpr int cpuFreq = F_CPU;
+  UNUSED(cpuFreq);
+
   #if ENABLED(SDSUPPORT) && DISABLED(SDIO_SUPPORT) && (defined(SDSS) && SDSS != -1)
     OUT_WRITE(SDSS, HIGH); // Try to set SDSS inactive before any other SPI users start up
   #endif

Marlin/src/HAL/STM32/inc/Conditionals_adv.h

 #if defined(USBD_USE_CDC_MSC) && DISABLED(NO_SD_HOST_DRIVE)
   #define HAS_SD_HOST_DRIVE 1
 #endif
+
+// Fix F_CPU not being a compile-time constant in STSTM32 framework
+#ifdef BOARD_F_CPU
+  #undef F_CPU
+  #define F_CPU BOARD_F_CPU
+#endif

buildroot/share/PlatformIO/scripts/common-cxxflags.py

   #"-Wno-sign-compare"
 ])
 
+#
+# Add CPU frequency as a compile time constant instead of a runtime variable
+#
+def add_cpu_freq():
+	if 'BOARD_F_CPU' in env:
+		env['BUILD_FLAGS'].append('-DBOARD_F_CPU=' + env['BOARD_F_CPU'])
+
 # Useful for JTAG debugging
 #
 # It will separe release and debug build folders.
 #
 if env.GetBuildType() == "debug":
 	env['BUILD_DIR'] = '$PROJECT_BUILD_DIR/$PIOENV/debug'
+
+# On some platform, F_CPU is a runtime variable. Since it's used to convert from ns 
+# to CPU cycles, this adds overhead preventing small delay (in the order of less than 
+# 30 cycles) to be generated correctly. By using a compile time constant instead
+# the compiler will perform the computation and this overhead will be avoided
+add_cpu_freq()