[gdb/testsuite] Fix gdb.ada/scalar_storage.exp on s390x

[gdb/testsuite] Fix gdb.ada/scalar_storage.exp on s390x

On s390x-linux, with test-case gdb.ada/scalar_storage.exp we have:
...
(gdb) print V_LE^M
$1 = (value => 126, another_value => 12, color => 3)^M
(gdb) FAIL: gdb.ada/scalar_storage.exp: print V_LE
print V_BE^M
$2 = (value => 125, another_value => 9, color => green)^M
(gdb) KFAIL: $exp: print V_BE (PRMS: DW_AT_endianity on enum types)
...

The KFAIL is incorrect in the sense that gdb is behaving as expected.

The problem is incorrect debug info, so change this into an xfail.

Furthermore, extend the xfail to cover V_LE.

Tested on s390x-linux and x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>
PR testsuite/32875
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32875