Fix IAR compiler warnings

Two warnings have been fixed:
1. code 'if( len <= 0xFFFFFFFF )' gave warning 'pointless integer comparison'.
   This was fixed by wraping the condition in '#if SIZE_MAX > 0xFFFFFFFF'.
2. code 'diff |= A[i] ^ B[i];' gave warning 'the order of volatile accesses is undefined in'.
   This was fixed by read the volatile data in temporary variables before the computation.

Explain IAR warning on volatile access

Consistent use of CMAKE_C_COMPILER_ID