IntelliSense auto-generation does not add correct -mmcu compiler argument

[dumbasPL]

As requested here I would like to reopen this issue.

MCU-specific code like inline assembly or direct register access will generate false-positive error messages in IntelliSense.

Example of problematic code:

//example 1: PRGMEM access
//based on https://www.arduino.cc/reference/en/language/variables/utilities/progmem/
const char signMessage[] PROGMEM = {"This is a test"};
char myChar;
void foo() {
  // read back a char
  for (byte k = 0; k < strlen_P(signMessage); k++) {
    myChar = pgm_read_byte_near(signMessage + k); // this will throw `unknown register name 'r0' C/C++(1118)` but compile just fine
    Serial.print(myChar);
  }
  Serial.println();
}

//example 2: register access
//based on https://www.arduino.cc/en/Reference/PortManipulation
void bar() {
  // sets Arduino pins 1 to 7 as outputs, pin 0 as input
  DDRD = B11111110; // this will throw `identifier "DDRD" is undefined C/C++(20)` but compile just fine

  // sets digital pins 7,5,3 HIGH
  PORTD = B10101000; // this will throw `identifier "PORTD" is undefined C/C++(20)` but compile just fine
}

Proposed solution add correct -mmcu flag based on currently selected board.
Example for an Arduino UNO board: -mmcu=atmega328p

Adding this flag solves all the above-mentioned issues.

[egcris]

Duplicate of #1279

vscode-arduino uses cocopa to parse Arduino's avr-g++ command-line and get the defines.

But cocopa doesn't include -mmcu (nor other flags) in the search scope for defines, as per elektronikworkshop/cocopa#7 :(

Pull request #8 at cocopa that solves this, still awaiting approval...