sim-se: Add default to SyscallDesc constructor
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 <5c8aa1266c03186393d9e7b3b4e3b14239af8201@google.com>
Maintainer: Brandon Potter <0c4047814a967fe28855a3d9d73d9b4959444c57@amd.com>

TUD-OS/gem5-dtu,TUD-OS/gem5-dtu,gem5/gem5,gem5/gem5,TUD-OS/gem5-dtu,TUD-OS/gem5-dtu,gem5/gem5,TUD-OS/gem5-dtu,gem5/gem5,gem5/gem5,gem5/gem5,TUD-OS/gem5-dtu,gem5/gem5,TUD-OS/gem5-dtu