runtime: emit more specific errors from checkptr

runtime: emit more specific errors from checkptr

Update error messages for pointer alignment checks and pointer
arithmetic checks so that each type of error has a unique error
message.

Fixes #37488

Change-Id: Ida2c2fa3f041a3307d665879a463f9e8f2c1fd03
Reviewed-on: https://go-review.googlesource.com/c/go/+/223037
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>