This commit fixes and improves the generation of dynamic symbols for
XCOFF files.
This mainly adds for every dynamic symbols a new symbol named
s.Extname().

Change-Id: I5b788f076d9a05e5d42f08eb1a74fd3e3efa9a86
Reviewed-on: https://go-review.googlesource.com/c/151038
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>