[lld/mac] On Apple systems, call CC_SHA256 from libSystem

It's in libSystem, so it doesn't bring in any new deps, and it's
currently much faster than LLVM's current SHA256 implementation.

Makes linking (arm64) Chromium Framework with ld64.lld 17% faster.
See also PR56121.

No behavior change.

Differential Revision: https://reviews.llvm.org/D128290