arch,base,cpu,sim: Statically allocate debugSymbolTable.

arch,base,cpu,sim: Statically allocate debugSymbolTable.

This singleton object is used thruoughout the simulator. There is
really no reason not to have it statically allocated, except that
whether it was allocated seems to sometimes be used as a signal that
something already put symbols in it, specifically in SE mode.

To keep that functionality for the moment, this change adds an "empty"
method to the SymbolTable class to make it easy to check if the symbol
table is empty, or if someone already populated it.

Change-Id: Ia93510082d3f9809fc504bc5803254d8c308d572
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/24785
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>