Any defer in a shared object crashed when GOARCH=386. This turns out to be two
bugs:

 1) Calls to morestack were not processed to be PIC safe (must have been
    possible to trigger this another way too)
 2) jmpdefer needs to rewind the return address of the deferred function past
    the instructions that load the GOT pointer into BX, not just past the call

Bug 2) requires re-introducing the a way for .s files to know when they are
being compiled for dynamic linking but I've tried to do that in as minimal
a way as possible.

Fixes #15916

Change-Id: Ia0d09b69ec272a176934176b8eaef5f3bfcacf04
Reviewed-on: https://go-review.googlesource.com/23623
Run-TryBot: Michael Hudson-Doyle <michael.hudson@canonical.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>