scons: revert removal of -Werror for gem5 21.0

This reverts:
https://gem5-review.googlesource.com/c/public/gem5/+/43425

Change-Id: Ic239150a7f2968744e40df40a6c03a942dc41ea6
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/43646
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>

SConstruct

@@ -322,6 +322,12 @@ if main['GCC'] or main['CLANG']:
     if GetOption('gold_linker'):
         main.Append(LINKFLAGS='-fuse-ld=gold')
 
+    # Treat warnings as errors but white list some warnings that we
+    # want to allow (e.g., deprecation warnings).
+    main.Append(CCFLAGS=['-Werror',
+                         '-Wno-error=deprecated-declarations',
+                         '-Wno-error=deprecated',
+                        ])
 else:
     error('\n'.join((
           "Don't know what compiler options to use for your compiler.",