runtime: tighten systemstack in lock assertions

We use systemstack on the locking path to avoid stack splits which could
cause locks to be recorded out of order (see comment on lockWithRank).

This concern is irrelevant on lock assertions, where we simply need to
see if a lock is held and don't care if another is taken in the
meantime. Thus we can simply drop these unless we actually need to
crash.

Updates #40677

Change-Id: I85d730913a59867753ee1ed0386f8c5efda5c432
Reviewed-on: https://go-review.googlesource.com/c/go/+/266718


Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Trust: Michael Pratt <mpratt@google.com>