Correct use of __FreeBSD_kernel__

It is defined on recent FreeBSD versions, so must not be mutually
exclusive with an #elif FreeBSD block.

Patch submitted by Robert Millan.

Fixes PR#16447.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@184867 91177308-0d34-0410-b5e6-96231b3b80d8