cores: add verilog_utils core

The idea behind the verilog_utils core is to gather convinience
functions in one place, that other cores can pull in and use.
As a start, it contains an implementation of clog2, for
tools that doesn't yet have proper support for $clog2
(most notable ISE < 14.1).
Also functions for find first/last one are included.
wb_intercon: use verilog_utils

Takes the recently added verilog_utils into use,
which make the implementation slightly cleaner.
By this, a bug in the $clog2 implementation in ISE < 14.1
is resolved.
And a workaround for a bug in verilator is removed
(more recent version of verilator than where the bug was
present, is needed for other reasons anyway).