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

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>