curl_setup.h: add support for IAR compiler

[josepho0918]

Enable some supported GCC attributes for IAR compiler.

[bagder]

./curl_setup.h:294:86: warning: Longer than 79 columns (LONGLINE)
 #if (defined(__GNUC__) || defined(__clang__) || defined(__IAR_SYSTEMS_ICC__)) &&    \
./curl_setup.h:295:86: warning: Longer than 79 columns (LONGLINE)
   defined(__STDC_VERSION__) && (__STDC_VERSION__ >= 199901L) &&                     \
./curl_setup.h:724:97: warning: Longer than 79 columns (LONGLINE)
 #if (defined(__GNUC__) && (__GNUC__ >= 3)) || defined(__clang__) || defined(__IAR_SYSTEMS_ICC__)

[bagder]

Thanks!