F_CPU define not picked up correctly in Arduino IDE #3098

Describe the bug
Attempting to use F_CPU for device detection doesn't work. It seems that F_CPU is actually defined later in the build process, so cannot be picked up properly using defines.

To Reproduce

#if defined F_CPU
  #if F_CPU >= 45000000
    #define bufferSize 1000
  #else
    #define bufferSize 100
  #endif

#else
  #define bufferSize 500
#endif

void setup() {
    Serial.begin(115200);
}

void loop() {
  Serial.print("Buffer Size: ");
  Serial.println(bufferSize);
  Serial.print("F_CPU: ");
  Serial.println(F_CPU);
  delay(1000);
}

Steps to reproduce the behavior:

1. Run the included example code
2. See F_CPU is displayed as 96MHz, although the selected buffer size is the smallest (detected at <45Mhz)

Expected behavior
Expected F_CPU to actually detect the configured clock speed, and allow configuration of buffer sizes based on CPU speed.

Desktop (please complete the following information):

• OS: Windows 11
• Arduino IDE version: 2.3.10
• STM32 core version: 3.0.0
• Tools menu settings if not the default: Any
• Upload method: DFU

Board (please complete the following information):

• Name: Black Pill STM32F411CEU6
• Hardware Revision: n/a
• Extra hardware used if any: none required

Additional context
See nRF24/RF24Network#278
nRF24/RF24Network#277