scons: Disable Address Sanitizer for GCC (#951)

This removes the '--with-asan' as it's are not compatible with GCC. The
`--with-asan` incompatibility with GCC is discussed in Issue #916.

SConstruct

@@ -707,10 +707,16 @@ for variant_path in variant_paths:
     if GetOption('with_ubsan'):
         sanitizers.append('undefined')
     if GetOption('with_asan'):
-        # Available for gcc >= 5 or llvm >= 3.1 both a requirement
-        # by the build system
+        if env['GCC']:
+            # Address sanitizer is not supported with GCC. Please see Github
+            # Issue https://github.com/gem5/gem5/issues/916 for more details.
+            warning("Address Sanitizer is not supported with GCC. "
+                    "This option will be ignored.")
+        else:
+            # Available for llvm >= 3.1. A requirement by the build system.
             sanitizers.append('address')
-        suppressions_file = Dir('util').File('lsan-suppressions').get_abspath()
+            suppressions_file = Dir('util').File('lsan-suppressions')\
+                                .get_abspath()
             suppressions_opt = 'suppressions=%s' % suppressions_file
             suppressions_opts = ':'.join([suppressions_opt,
                                         'print_suppressions=0'])