On Intel Core i7, I got

[hjl@gnu-6 997]$ cat saved.c
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mman.h>

int main()
{
    char *buf, *buf2;

    buf = (char*)mmap((void*)0x100000000, 0x2000 * 2,
                      PROT_NONE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
    buf = mmap((char *)buf, 0x2000, PROT_READ | PROT_WRITE,
               MAP_PRIVATE | MAP_ANONYMOUS | MAP_FIXED, -1, 0);
    memset(buf, 'a', 0x2000);
    buf[0x1fff] = 0;
    buf2 = strdup(buf);
    if (strncmp (buf + 3994, buf2 + 2635, 6241) >= 0)
      abort ();

    return 0;
}
[hjl@gnu-6 997]$ gcc saved.c
[hjl@gnu-6 997]$ ./a.out 
Segmentation fault
[hjl@gnu-6 997]$ gdb a.out 
GNU gdb (GDB) 7.2.50.20110312-cvs
Copyright (C) 2011 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-unknown-linux-gnu".
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>...
Reading symbols from /export/home/hjl/bugs/libc/997/a.out...(no debugging symbols found)...done.
(gdb) r
Starting program: /export/home/hjl/bugs/libc/997/a.out 

Program received signal SIGSEGV, Segmentation fault.
0x0000003f7d3214a0 in __strncmp_sse42 () from /lib64/libc.so.6
(gdb)