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

[Martin Storsjö via llvm-commits]

mstorsjo 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.

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