[llvm] [JITLink][Cygwin] undef i386 in JITLink/i386.h (PR #138218)

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jeremyd2019 wrote:

> > I'm betting it's OK now, because we already have to define `_GNU_SOURCE` to get `dladdr`. BTW, is this the cmake file where it'd make sense to do that, rather than requiring the builder to specify it in CFLAGS?
> 
> Not entirely sure offhand; there's a couple of different ones in `llvm/cmake` that set various options - I'd grep around and see if we set something similar somewhere, and add it where it seems to fit in.

#138329

https://github.com/llvm/llvm-project/pull/138218