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

[Raul Tambre via llvm-commits]

tambry 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.

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