sim-se: Add default to SyscallDesc constructor

This patch adds a default value of unimplementedFunc to SyscallDesc's
constructor's second argument, which makes adding system call
placeholders to SE mode cleaner.

Change-Id: I3e7efb3a887cc52d16d2095d3dd2e2a96fb882fb
Reviewed-on: https://gem5-review.googlesource.com/5501
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Brandon Potter <Brandon.Potter@amd.com>

src/sim/syscall_desc.hh

@@ -51,9 +51,13 @@
 #include "base/types.hh"
 
 class Process;
+class SyscallDesc;
 class SyscallReturn;
 class ThreadContext;
 
+SyscallReturn unimplementedFunc(SyscallDesc *desc, int num,
+    Process *p, ThreadContext *tc);
+
 /**
  * This class provides the wrapper interface for the system call
  * implementations which are defined in the sim/syscall_emul files and
@@ -66,7 +70,8 @@ class SyscallDesc {
     typedef SyscallReturn (*SyscallExecutor)(SyscallDesc*, int num,
                                              Process*, ThreadContext*);
 
-    SyscallDesc(const char *name, SyscallExecutor sys_exec, int flags = 0)
+    SyscallDesc(const char *name,
+                SyscallExecutor sys_exec=unimplementedFunc, int flags=0)
         : _name(name), executor(sys_exec), _flags(flags), _warned(false)
     {
     }