[gdb/testsuite] Fix gdb.dwarf2/shortpiece.exp on s390x

On s390x-linux, I run into:
...
(gdb) p (short []) s1^M
$3 = {0, 1, 0, <optimized out>}^M
(gdb) FAIL: gdb.dwarf2/shortpiece.exp: p (short []) s1
...
while this is expected:
...
(gdb) p (short []) s1^M
$3 = {1, 0, 0, <optimized out>}^M
(gdb) PASS: gdb.dwarf2/shortpiece.exp: p (short []) s1
...

The type of s1 is:
...
(gdb) ptype s1
type = struct S {
    myint a;
    myushort b;
}
...
so the difference is due the fact that viewing an int as two shorts gives
different results depending on the endianness.

Fix this by allowing both results.

Tested on x86_64-linux and s390x-linux.

Approved-By: Tom Tromey <tom@tromey.com>