systemc: Add the nonstandard sc_user exception type.

This type is not in the spec but is used in the tests.

Change-Id: I4537a33b0b5dcb3c72a091276d601c244c4a3862
Reviewed-on: https://gem5-review.googlesource.com/11284
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

systemc: Stub out all the standard utilility classes and functions.

Change-Id: I9e9724edb6281e0b0a6bae5546b0ede77d295c12
Reviewed-on: https://gem5-review.googlesource.com/10841
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>