systemc: Implement sc_get_curr_process_handle().

The implementation is fairly trivial, but the pieces it relies on
weren't always available.

Change-Id: Ie77168ff336febd3ebe22c400f2b1f133e43aca7
Reviewed-on: https://gem5-review.googlesource.com/c/12973
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

src/systemc/core/sc_process_handle.cc

@@ -70,8 +70,7 @@ sc_set_location(const char *file, int lineno)
 sc_process_b *
 sc_get_curr_process_handle()
 {
-    warn("%s not implemented.\n", __PRETTY_FUNCTION__);
-    return nullptr;
+    return ::sc_gem5::scheduler.current();
 }