base: Defining make_unique for C++11

base: Defining make_unique for C++11

std::make_unique is not available for C++11 compilers, and it has been
introduced only in C++14.  Since gem5 is not officially supporting the
latter at the moment, this patch allows to use it in gem5 if including
base/compiler.hh. If compiled under C++14, std::make_unique will be
used instead.

Change-Id: Ibf1897fad0a1eb1cb0c683cc25170feaa6841997
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/5201
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>