[gdb/testsuite] Fix gdb.python/py-value-cc.exp for big endian

[gdb/testsuite] Fix gdb.python/py-value-cc.exp for big endian

On s390x-linux, I run into:
...
(gdb) python print(u[u_fields[0]])^M
99^M
(gdb) PASS: gdb.python/py-value-cc.exp: u's first field via field
python print(u[u_fields[1]])^M
0 '\000'^M
(gdb) FAIL: gdb.python/py-value-cc.exp: u's second field via field
...

There's a var u of this type:
...
union U {
  int a;
  char c;
};
...
and after assigning 99 to u.a, the test-case expects u.c to contain 99 (which
it does on x86_64), but instead it contains 0.

Fix this by instead assigning 0x63636363, to ensure that u.c == 99 for both
little and big endian.

Tested on x86_64-linux and s390x-linux.