hsail-x86: fix gpu dynamic instruction error

hsail-x86: fix gpu dynamic instruction error

The gpu_dyn_inst.hh file was missing a clone method from
inherited classes. (The clone method is the way to implement
the prototype design pattern.) Because the inherited clone
method was declare as pure virtual, the method needed to
be implemented. Otherwise, the compiler complains that the
class is abstract.

Change-Id: I38782d5f7379f32be886401f7c127fe60d2f8811
Reviewed-on: https://gem5-review.googlesource.com/12108
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>
Maintainer: Anthony Gutierrez <anthony.gutierrez@amd.com>