The locations chosen for racewalking inserted code can
be wrong and thus cause unwanted next/step behavior in
debuggers.  Forcing the positions to be unset results in
better behavior.

Test added, and test harness corrected to deal with
changes to gdb's output caused by -racewalk.

Incidental changes in Delve (not part of the usual testing,
but provided because we care about Delve) also reflected
in this CL.

Fixes #22600.

Change-Id: Idd0218afed52ab8c68efd9eabbdff3c92ea2b996
Reviewed-on: https://go-review.googlesource.com/78336
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: Alessandro Arzilli <alessandro.arzilli@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>