[OpenMP] Provide big-endian bitfield definitions (#69995)

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.