systemc: Replace sc_core::sc_type_index with std::type_index.

systemc: Replace sc_core::sc_type_index with std::type_index.

The former is either the same as the later, or a custom wrapper if
C++11 isn't supported. Since we generally expect C++11 support, we can
skip the indirection.

Change-Id: I9a45e3854bb7cc56d094e3fe2773fe4b5c94403b
Reviewed-on: https://gem5-review.googlesource.com/c/15297
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>
Maintainer: Anthony Gutierrez <anthony.gutierrez@amd.com>