Enable LLDB by default on amd64 and arm64

LLDB is usable for userland core file and live debugging on amd64, and
for userland core file debugging on arm64. In general it works at least
as well on FreeBSD as our in-tree gdb version, so enable it by default
to allow for broader use and testing.

An LLDB tutorial is available at http://lldb.llvm.org/tutorial.html, and
a table mapping GDB commands to LLDB commands can be found at
http://lldb.llvm.org/lldb-gdb.html .

LLDB also has some level of support for FreeBSD on arm, mips, i386,
and powerpc, but is not yet ready to have them enabled by default.

Reviewed by: gnn
Relnotes: Yes