util: Further consolidate the Args interface in the m5 utility.

util: Further consolidate the Args interface in the m5 utility.

Create static methods to convert any string to an integer or to pack it
into an array of integers. Create non-static methdos named pop() to
pop() the first element and simultaneously convert it. If the conversion
fails, the argument is not popped.

Change-Id: I55d98b3971e7abb7b6206d06ec7fcf046e828d29
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27553
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>