[OpenMP] fix endianness dependent definitions in OMP headers for MSVC (#84540)

MSVC does not define __BYTE_ORDER__ making the check for BigEndian
erroneously evaluate to true and breaking the struct definitions in MSVC
compiled builds correspondingly. The fix adds an additional check for
whether __BYTE_ORDER__ is defined by the compiler to fix these.

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Co-authored-by: Vadim Paretsky <b-vadipa@microsoft.com>