Variable lookup is slow because it requires a full scan of debug_info
to check for package variables, this doesn't matter much in interactive
use but can slow down evaluation of breakpoint conditions
significantly.

Providing benchmark proof for this is hard since this effect doesn't
show for small programs with small debug_info sections.

When a Go program is externally linked, the external linker is
responsible for picking the TLS offset. It records its decision in the
runtime.tlsg symbol. Read the offset from that rather than guessing -16.

This implementation causes a regression: 1.4 and earlier will no longer
work.

- move native backend to pkg/proc/native
- move gdbserver backend to pkg/proc/gdbserial
- move core dumps backend to pkg/proc/core

* proc: refactor BinaryInfo part of proc.Process to own type

The data structures and associated code used by proc.Process
to implement target.BinaryInfo will also be useful to support a
backend for examining core dumps, split this part of proc.Process
to a different type.

* proc: compile support for all executable formats unconditionally

So far we only compiled in support for loading the executable format
supported by the host operating system.
Once support for core files is introduced it is however useful to
support loading in all executable formats, there is no reason why it
shouldn't be possible to examine a linux coredump on windows, or
viceversa.

* proc: bugfix: do not resume threads on detach if killing

* Replace BinaryInfo interface with BinInfo() method returning proc.BinaryInfo