[gdb/testsuite] Fix gdb.ada/dyn-bit-offset.exp on s390x

[gdb/testsuite] Fix gdb.ada/dyn-bit-offset.exp on s390x

On s390x-linux, with test-case gdb.ada/dyn-bit-offset.exp and gcc 7.5.0 I get:
...
(gdb) print spr^M
$1 = (discr => 3, array_field => (-5, -6, -7), field => -6, another_field => -6)^M
(gdb) FAIL: $exp: print spr
print spr.field^M
$2 = -6^M
(gdb) FAIL: $exp: print spr.field
...

On x86_64-linux, with the same compiler version I get:
...
(gdb) print spr^M
$1 = (discr => 3, array_field => (-5, -6, -7), field => -4, another_field => -4)^M
(gdb) XFAIL: $exp: print spr
print spr.field^M
$2 = -4^M
(gdb) PASS: $exp: print spr.field
...

In both cases, we're hitting the same compiler problem, but it manifests
differently on little and big endian.

Make sure the values seen for both little and big endian trigger xfails
for both tests.

Printing spr.field gives the expected value -4 for x86_64, but that's an
accident.  Change the actual spr.field value to -5, to make sure
that we get the same number of xfails on x86_64 and s390x.

Finally, make the xfails conditional on the compiler version.

Tested using gcc 7.5.0 on both x86_64-linux and s390x-linux.

Approved-By: Andrew Burgess <aburgess@redhat.com>
PR testsuite/33042
https://sourceware.org/bugzilla/show_bug.cgi?id=33042