[gdb/testsuite] Fix PR20630 regression test in gdb.base/printcmds.exp

On s390x-linux, I run into:
...
(gdb) print {unsigned char}{65}^M
$749 = 0 '\000'^M
(gdb) FAIL: gdb.base/printcmds.exp: print {unsigned char}{65}
...

In contrast, on x86_64-linux, we have:
...
(gdb) print {unsigned char}{65}^M
$749 = 65 'A'^M
(gdb) PASS: gdb.base/printcmds.exp: print {unsigned char}{65}
...

The first problem here is that the test is supposed to be a regression test
for PR20630, which can be reproduced (for an unfixed gdb) like this:
...
(gdb) p {unsigned char[]}{0x17}
gdbtypes.c:4641: internal-error: copy_type: \
  Assertion `TYPE_OBJFILE_OWNED (type)' failed.
...
but it's not due to insufficient quoting (note the dropped '[]').

That's easy to fix, but after that we have on s390 (big endian):
...
(gdb) print {unsigned char[]}{65}^M
$749 = ""^M
...
and on x86_64 (little endian):
...
(gdb) print {unsigned char[]}{65}^M
$749 = "A"^M
...

Fix this by using 0xffffffff, such that in both cases we have:
...
(gdb) print {unsigned char[]}{0xffffffff}^M
$749 = "\377\377\377\377"^M
...

Tested on x86_64-linux and s390x-linux.