misc: Revert v21.2 specific commits for develop

These changes were made to the release staging branch for the v21.2
release. This commit reverts them for the develop branch.

Change-Id: I9f02470d00d5034a0797f32d4c1fe0e7055860a6
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/54904
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>

SConstruct

@@ -363,6 +363,13 @@ if main['GCC'] or main['CLANG']:
                 conf.CheckLinkFlag(
                         '-Wl,--thread-count=%d' % GetOption('num_jobs'))
 
+    # 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.",