sim: cgen: namespace custom trace functions

sim: cgen: namespace custom trace functions

The cgen code declares some macros/funcs using the trace_xxx prefix, but
the code isn't generic and only works with cgen targets.  This is blocking
the creation of new common trace functions.

Let's blindly add cgen_xxx prefixes to all these symbols.  Some already
use this convention to avoid conflicts, so it makes sense to align them.
In the future we might want to move some to the common trace core, but
one thing at a time.