systemc: Make verify.py ignore another warning.
systemc: Make verify.py ignore another warning.

Also make warning filters more general and easier to add by writing a
function to generate them.

Change-Id: I8d813ded9ad8a9ccac4e48e5ea80a3c9c23959da
Reviewed-on: https://gem5-review.googlesource.com/12061
Reviewed-by: Gabe Black <5c8aa1266c03186393d9e7b3b4e3b14239af8201@google.com>
Maintainer: Gabe Black <5c8aa1266c03186393d9e7b3b4e3b14239af8201@google.com>

gem5/gem5,gem5/gem5,gem5/gem5,TUD-OS/gem5-dtu,gem5/gem5,TUD-OS/gem5-dtu,gem5/gem5,TUD-OS/gem5-dtu,TUD-OS/gem5-dtu,TUD-OS/gem5-dtu,gem5/gem5,TUD-OS/gem5-dtu,TUD-OS/gem5-dtu,gem5/gem5