[OpenMP] Provide big-endian bitfield definitions

[iii-i]

structs kmp_depend_info.flags and kmp_tasking_flags contain bitfields, which overlay integer flag values. The current bitfield definitions target little-endian machines. On big-endian machines bitfields are laid out in the opposite order, so the current definitions do not work there.

There are two ways to fix this: either provide big-endian bitfield definitions, or bit-swap integer flag values. Go with the former, since it's localized to one place and therefore is more maintainable.

[shiltian]

I'm wondering since we use them via x.y, does it really affect the correctness on big-endian systems? I understand given current implementation, unused or reserved will be at the beginning of a buffer.

[iii-i]

If we were doing just that, it would be totally fine. However, there is code that bitcasts ints to these structs like this:

kmp_task_t *__kmpc_omp_task_alloc(ident_t *loc_ref, kmp_int32 gtid,
                                  kmp_int32 flags, size_t sizeof_kmp_task_t,
                                  size_t sizeof_shareds,
                                  kmp_routine_entry_t task_entry) {
  kmp_task_t *retval;
  kmp_tasking_flags_t *input_flags = (kmp_tasking_flags_t *)&flags;

That's where big- and little-endian differ.

[shiltian]

Oh, right. Yeah, we need that.