scons: Increase the minimum version of clang to 6.

Change-Id: I41f7e96922c273827c72df06145f0fe433936b59
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/45621
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>

SConstruct

@@ -376,8 +376,8 @@ if main['GCC']:
                                   '-fno-builtin-realloc', '-fno-builtin-free'])
 
 elif main['CLANG']:
-    if compareVersions(main['CXXVERSION'], "3.9") < 0:
-        error('clang version 3.9 or newer required.\n'
+    if compareVersions(main['CXXVERSION'], "6") < 0:
+        error('clang version 6 or newer required.\n'
               'Installed version:', main['CXXVERSION'])
 
     # Set the Link-Time Optimization (LTO) flags if enabled.