[llvm] [Offload] define MSVC byte-order macros for rtl endianness checks (PR #202540)

[Weronika Lewandowska via llvm-commits]

wlemkows wrote:

@jhuber6 @Meinersbur @adurang Thank you for sharing your thoughts. In that case, now that we have dependencies in liboffload, should we switch to llvm ADT in this PR or stick with the current solution until ADT runtimes are available?

https://github.com/llvm/llvm-project/pull/202540