misc,sim: Fixed std::array bracket compiler error

misc,sim: Fixed std::array bracket compiler error

For versions of Clang before 6.0, Clang returns an error if and
std::array initialization is not encompassed in two sets of
encompassing braces. This is a known compiler bug:
https://bugs.llvm.org/show_bug.cgi?id=21629.

As we support Clang 3.9 onwards, we are required to include these
redundant braces to ensure compilation. They do not produce any
ill-effects when using later clang compilers or with any GCC compiler
gem5 presently supports.

Change-Id: Ia512a9b9f583b1cfa28f9fc4c24f6e202e46b4cb
Issue-on: https://gem5.atlassian.net/browse/GEM5-563
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/29294
Reviewed-by: Pouya Fotouhi <pfotouhi@ucdavis.edu>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>