runtime: handle end PC in textAddr

As the func table contains the end marker of the text section, we
sometimes need to get that address from an offset. Currently
textAddr doesn't handle that address, as it is not within any
text section. Instead of letting the callers not call textAddr
with the end offset, just handle it more elegantly in textAddr.

For #48837.

Change-Id: I6e97e455f6cb66e9680a7aac6152ba6f4cda2e12
Reviewed-on: https://go-review.googlesource.com/c/go/+/354635


Trust: Cherry Mui <cherryyz@google.com>
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>