[lldb] [Process/FreeBSDKernel] Introduce libkvm support

Introduce initial support for using libkvm on FreeBSD.  The library
can be used as an alternate implementation for processing kernel
coredumps but it can also be used to access live kernel memory through
specifying "/dev/mem" as the core file, i.e.:

    lldb --core /dev/mem /boot/kernel/kernel

Differential Revision: https://reviews.llvm.org/D116005