systemc: fix -Wno-free-nonheap-object for building scheduler.cc

-Wno-free-nonheap-object can happen at compile or link time depending on
the versions. To better disable this false alarm, we move the memory
management part into .cc file, so the check is always done at link time.

This change also removes the global flags so other code is still checked
with the flags.

Change-Id: I8f1e20197b25c90b5f439e2ecc474bd99e4f82ed
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/67237
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Yu-hsin Wang <yuhsingw@google.com>
Maintainer: Gabe Black <gabe.black@gmail.com>

SConstruct

@@ -447,10 +447,6 @@ for variant_path in variant_paths:
             error('gcc version 7 or newer required.\n'
                   'Installed version:', env['CXXVERSION'])
 
-        with gem5_scons.Configure(env) as conf:
-            # This warning has a false positive in the systemc in g++ 11.1.
-            conf.CheckCxxFlag('-Wno-free-nonheap-object')
-
         # Add the appropriate Link-Time Optimization (LTO) flags if
         # `--with-lto` is set.
         if GetOption('with_lto'):