Reportedly on some new Fedora systems the linker is producing extra
load segments, basically making the dynamic section non-executable.
We were assuming that the first load segment could be used to
determine the program's load offset, but that is no longer true.
Use the first executable load segment instead.

Fixes #26369

Change-Id: I5ee31ddeef2e8caeed3112edc5149065a6448456
Reviewed-on: https://go-review.googlesource.com/127895
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>