sim: Fix a bug in prlimit syscall in SE mode

The old_limit pointer is supposed to be the 4th argument (index 3) of
the prlimit syscall. This patch sets old_limit pointer to the correct
argument.

Change-Id: I97808f7234cd2622cb3eb2f1e0beb7fc8cf492c1
Reviewed-on: https://gem5-review.googlesource.com/6903
Reviewed-by: Alec Roelke <ar4jc@virginia.edu>
Maintainer: Brandon Potter <Brandon.Potter@amd.com>

src/sim/syscall_emul.hh

@@ -1705,8 +1705,7 @@ prlimitFunc(SyscallDesc *desc, int callnum, Process *process,
     Addr o = process->getSyscallArg(tc, index);
     if (o != 0)
     {
-        TypedBufferArg<typename OS::rlimit> rlp(
-                process->getSyscallArg(tc, index));
+        TypedBufferArg<typename OS::rlimit> rlp(o);
         switch (resource) {
           case OS::TGT_RLIMIT_STACK:
             // max stack size in bytes: make up a number (8MB for now)