runtime: make funcspdelta inlineable

runtime: make funcspdelta inlineable

funcspdelta should be inlined: It is a tiny wrapper around another func.
The sanity check prevents that. Condition the sanity check on debugPcln.
While we're here, make the sanity check throw when it fails.

Change-Id: Iec022b8463b13a8e5a6d8479e7ddcb68909d6fe0
Reviewed-on: https://go-review.googlesource.com/c/go/+/354133
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>