Treat warnings as errors for IAR

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
diff --git a/library/asn1write.c b/library/asn1write.c
index c01c836..72acdf3 100644
--- a/library/asn1write.c
+++ b/library/asn1write.c
@@ -83,7 +83,9 @@
         return( 4 );
     }
 
+#if SIZE_MAX > 0xFFFFFFFF
     if( len <= 0xFFFFFFFF )
+#endif
     {
         if( *p - start < 5 )
             return( MBEDTLS_ERR_ASN1_BUF_TOO_SMALL );
@@ -96,7 +98,9 @@
         return( 5 );
     }
 
+#if SIZE_MAX > 0xFFFFFFFF
     return( MBEDTLS_ERR_ASN1_INVALID_LENGTH );
+#endif
 }
 
 int mbedtls_asn1_write_tag( unsigned char **p, unsigned char *start, unsigned char tag )