[libc++] NFC: Reindent the feature test macro generation script

Each feature-test macro is now a clear block indentation-wise.

Cr-Mirrored-From: https://chromium.googlesource.com/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 7656dd33d9326df0b86f08676474dbf8055866b8
1 file changed