scons: "no-defautled-function-deleted" flag moved to only clang8+

It has been onserved that clang does not function with the
"-Wno-defaulted-function-deleted" for versions below clang8.

Change-Id: I6e6d1476350ae2b46b4de971fe3456697b39e43c
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/29454
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
diff --git a/SConstruct b/SConstruct
index 142800e..370cd60 100755
--- a/SConstruct
+++ b/SConstruct
@@ -475,11 +475,13 @@
                          # use struct hash and class hash
                          # interchangeably.
                          '-Wno-mismatched-tags',
-                         '-Wno-defaulted-function-deleted',
                          ])
     if compareVersions(clang_version, "10.0") >= 0:
         main.Append(CCFLAGS=['-Wno-c99-designator'])
 
+    if compareVersions(clang_version, "8.0") >= 0:
+        main.Append(CCFLAGS=['-Wno-defaulted-function-deleted'])
+
     main.Append(TCMALLOC_CCFLAGS=['-fno-builtin'])
 
     # On Mac OS X/Darwin we need to also use libc++ (part of XCode) as