tests/xstatx.c: use zero_extend_signed_to_ull/sign_extend_unsigned_to_ll

tests/xstatx.c: use zero_extend_signed_to_ull/sign_extend_unsigned_to_ll

Use zero_extend_signed_to_ull and sign_extend_unsigned_to_ll macros
instead of explicit casts with unpredictable sign extension semantics.

* tests/xstatx.c (print_time, main): Use zero_extend_signed_to_ull
instead of explicit cast.
(print_stat): Use zero_extend_signed_to_ull and
sign_extend_unsigned_to_ll instead of explicit casts.