[compiler-rt] [compiler-rt] Allow to build on CYGWIN (PR #66348)

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

Thanks, this looks good to me. For context; are you building compiler-rt with the default GCC of cygwin, or with clang targeting cygwin?

https://github.com/llvm/llvm-project/pull/66348