elf-core: Remove now-unused compile-time FreeBSD support

This change removes the final instances of compile-time #ifdef magic
from the elf core plugin.  Also rename the classes to ELFLinux... as
they're specific to Linux.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@186977 91177308-0d34-0410-b5e6-96231b3b80d8