systemc: Implement sc_gen_unique_name.

systemc: Implement sc_gen_unique_name.

The Accellera implementation statically allocates the buffer it uses to
build the unique names and only allocates the name generator if it's
going to be used for a particular module. I assume that's to avoid
allocating a large buffer if it's not going to be used.

In this implementation, I use an std::string which manages its own
memory and so shouldn't need to be selectively allocated. I also use a
string stream to construct the name instead of sprintf.

Change-Id: If92c68586a85b5d27c067a75a6e9ebbf00d8c785
Reviewed-on: https://gem5-review.googlesource.com/12066
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>