elf-core: Support FreeBSD at compile-time

Compile-time #ifdef-ery isn't right, but this makes core debugging work on
FreeBSD and highlights the parts that will need to be changed for runtime
arch support.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@186534 91177308-0d34-0410-b5e6-96231b3b80d8