[clang] Revert "[Fix] Disable fdefine-target-os-macros for now" (PR #78353)

[via cfe-commits]

zmodem wrote:

Could you add a release note about this, explaining how it changes the macros and what actions users should take?

We've already seen that it broke zlib builds, and we're having issues with libpng too in Chromium (https://crbug.com/1519899), so users are likely to run into this.

https://github.com/llvm/llvm-project/pull/78353