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

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

> @mstorsjo As far as I can tell `_DEFAULT_SOURCE` seems rather glibc and musl specific, which both are Linux-only. On FreeBSD these are enabled by default as these are mostly extensions originating from BSDs anyway. I expect the same to be true for other BSDs. For more exotic systems (e.g. Sun/Solaris) I think `_XOPEN_SOURCE` might be required but it's hard to verify. So I'd leave those up to others to discover.

Ok, fair enough.

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