[llvm] [Cygwin] CYGWIN is not WIN32 in current CMake (PR #143130)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-aarch64-sve2-vla` running on `linaro-g4-01` while building `llvm` at step 7 "ninja check 1".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/198/builds/5264

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 7 (ninja check 1) failure: 1200 seconds without output running [b'ninja', b'check-all'], attempting to kill
...
PASS: lld :: wasm/lto/thinlto.ll (97600 of 97610)
PASS: lld :: wasm/whole-archive.test (97601 of 97610)
PASS: lld :: MachO/stabs.s (97602 of 97610)
PASS: lld :: wasm/unresolved-symbols.s (97603 of 97610)
PASS: lld :: wasm/why-extract.s (97604 of 97610)
PASS: lld :: wasm/libsearch.s (97605 of 97610)
PASS: lld :: MachO/weak-definition-direct-fetch.s (97606 of 97610)
PASS: mlgo-utils :: corpus/extract_ir_test.py (97607 of 97610)
PASS: libFuzzer-aarch64-default-Linux :: value-profile-load.test (97608 of 97610)
PASS: lld :: MinGW/driver.test (97609 of 97610)
command timed out: 1200 seconds without output running [b'ninja', b'check-all'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=1539.382116

```

</details>

https://github.com/llvm/llvm-project/pull/143130