This adds a mechanism for debuggers to safely inject calls to Go
functions on amd64. Debuggers must participate in a protocol with the
runtime, and need to know how to lay out a call frame, but the runtime
support takes care of the details of handling live pointers in
registers, stack growth, and detecting the trickier conditions when it
is unsafe to inject a user function call.

Fixes #21678.
Updates derekparker/delve#119.

Change-Id: I56d8ca67700f1f77e19d89e7fc92ab337b228834
Reviewed-on: https://go-review.googlesource.com/109699
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>