stdlib, configs: Fix riscv-ubuntu-run error

This change,
https://gem5-review.googlesource.com/c/public/gem5/+/58790,
mistakenly constructs the root object outside of the
Simulator object, which would create another Root object.

The correct usage of a Simulator object is to pass the board to
the object without defining the root.

Change-Id: Iaa9aabe058a00abd964bf935b1b3f05456713275
Signed-off-by: Hoa Nguyen <hoanguyen@ucdavis.edu>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/58889
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
1 file changed