[compiler-rt] [cmake][compiler-rt] Define _DEFAULT_SOURCE instead of enabling extensions (PR #163377)

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

> @mstorsjo Changed over to explicit defines in the applicable files with comments for the functions its needed for.

This looks great, thanks! Are we sure that this is only needed on `__linux__` targets and nowhere else?

https://github.com/llvm/llvm-project/pull/163377