perf cputopo: Fix CPU topology reading on s/390

Commit fdf1e29b ("perf expr: Add metric literals for topology.")
fails on s390:

 # ./perf test -Fv 7
   ...
 # FAILED tests/expr.c:173 #num_dies >= #num_packages
   ---- end ----
   Simple expression parser: FAILED!
 #

Investigating this issue leads to these functions:
 build_cpu_topology()
   +--> has_die_topology(void)
        {
           struct utsname uts;

           if (uname(&uts) < 0)
                  return false;
           if (strncmp(uts.machine, "x86_64", 6))
                  return false;
           ....
        }

which always returns false on s390. The caller build_cpu_topology()
checks has_die_topology() return value. On false the
the struct cpu_topology::die_cpu_list is not contructed and has zero
entries. This leads to the failing comparison: #num_dies >= #num_packages.
s390 of course has a positive number of packages.

Fix this by adding s390 architecture to support CPU die list.

Output after:
 # ./perf test -Fv 7
  7: Simpl...