systemc: use warn_once for stack size operation

We have a warning in sc_spawn given that stack size setting is not supported.
However, as is a common call in SystemC, the warning generates too many
logs for users. This CL changes it to be warn_once.

Change-Id: I63c057ca99f68585303cf2b4fdddee5b713f856b
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/60750
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Reviewed-by: Yu-hsin Wang <yuhsingw@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/systemc/core/sc_spawn.cc

@@ -53,7 +53,7 @@ spawnWork(ProcessFuncWrapper *func, const char *name,
         if (opts->_dontInitialize)
             dontInitialize = true;
         if (opts->_stackSize != -1)
-            warn("Ignoring request to set stack size.\n");
+            warn_once("Ignoring request to set stack size.\n");
     }
 
     if (!name || name[0] == '\0') {