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

https://github.com/llvm/llvm-test-suite/pull/65 fixed the
llvm-test-suite errors. Reapply the change to enable
`fdefine-target-os-macros` by default for Darwin targets.

This reverts commit 63be986f612c175559efffed9daebcb944fa5cea.