[gdb/testsuite] Fix gdb.ada/big_packed_array.exp on s390x-linux

[gdb/testsuite] Fix gdb.ada/big_packed_array.exp on s390x-linux

When running test-case gdb.ada/big_packed_array.exp on s390x-linux, I run
into:
...
(gdb) print bad^M
$2 = (0 => 0 <repeats 24 times>, 1)^M
(gdb) FAIL: gdb.ada/big_packed_array.exp: scenario=minimal: print bad
...

This is with gcc 7.5.0, and this xfail should trigger:
...
           if { $have_xfail && [string is integer $last] \
                   && [expr ($last & 0xf) == 0] } {
                # gcc/101643
                setup_xfail *-*-*
            }
...
but it doesn't because $last is '1'.

Fix this by using 0xf0 as mask for big endian.

Tested on s390x-linux.