scons,tlm: Update the gem5-within-systemc SConstruct to use c++14.

Gem5 has moved to c++14, and the sources used from there will no longer
compile with an older version of the standard. Move this SConstruct from
specifying c++11 to c++14.

Change-Id: I5be151648c3b72b5cbc005acc0c208a11de458ba
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/42584
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>

util/tlm/SConstruct

@@ -55,7 +55,7 @@ env.Append(CPPPATH=[gem5_root + '/build/' + gem5_arch,
                     '#examples/common',
                     ])
 
-env.Append(CXXFLAGS=['-std=c++11',
+env.Append(CXXFLAGS=['-std=c++14',
                      '-DSC_INCLUDE_DYNAMIC_PROCESSES',
                      '-DTRACING_ON',
                      ])