panicwrap currently uses runtime.Callers and runtime.CallersFrames to
find the name of its caller. Simplify this by using getcallerpc.

This will be important for #16723, since to fix that we're going to
make CallersFrames skip the wrapper method, which is exactly what
panicwrap needs to see.

Change-Id: Icb0776d399966e31595f3ee44f980290827e32a6
Reviewed-on: https://go-review.googlesource.com/45411
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>