Add ops and lowering for variadic function calls

[robertoraggi]

Enough for

int printf(const char *format, ...);

int main() {
  char c = 'A';
  int i = 42;
  float f = 3.14f;
  double d = 2.71828;
  const char *str = "Hello, World!";

  printf("Character: %c\n", c);
  printf("Integer: %d\n", i);
  printf("Float: %.2f\n", f);
  printf("Double: %.5f\n", d);
  printf("String: %s\n", str);
}

$ cxx x.c -toolchain macos -c

$ ld x.o -syslibroot /Library/Developer/CommandLineTools/SDKs/MacOSX15.sdk -lSystem

$ ./a.out 
Character: A
Integer: 42
Float: 3.14
Double: 2.71828
String: Hello, World!