sim: Add function that returns all variables in a MathExpr

sim: Add function that returns all variables in a MathExpr

This changes adds support for retrieving all variables in a math
expression. The added function can be called in all valid expressions
and will return the variables in a vector of strings.

Change-Id: I086ba04aa1f798400c97a0b6bf982018a2457c64
Signed-off-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27889
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>