[compiler-rt] [compiler-rt] Avoid defining _KERNEL on FreeBSD (PR #143116)

[Jessica Clarke via llvm-commits]

jrtc27 wrote:

Hm, actually, we shouldn't even be trying to handle this (nor some others), they're Linux-specific and only defined so the Linuxulator can use them.

https://github.com/llvm/llvm-project/pull/143116