cpu: Use std::array for registers in SimpleThread.

cpu: Use std::array for registers in SimpleThread.

If the number of one of the register types is zero (useful on ARM in
the near future), memset will complain that it's given the length of
the array without multiplying by the size of the array elements. This
is a false positive since the length of the array and the number of
elements are both zero.

To avoid that warning/error and to simplify and update the SimpleThread
class slightly, this change replaces the C style arrays with
std::array.

Change-Id: Ifedd081a1940a578765c4d585e623236008ace67
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22523
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>