Currently the linker figures out where runtime-gdb.py should be by
looking for the path to runtime/debug.go. However, debug.go contains
only a few symbols and can easily get dead-code eliminated entirely,
especially from simple binaries. When this happens, the resulting
binary lacks a reference to runtime-gdb.py, so the GDB helpers don't
get loaded.

Fix this by instead sniffing for runtime/proc.go. This contains
runtime.main and the scheduler, so it's not going anywhere.

Change-Id: Ie3380c77c840d28614fada68b8c5861625f2aff5
Reviewed-on: https://go-review.googlesource.com/68019
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>