For shared libraries we need to be more flexible in how these symbols
are handled (e.g. sometimes tlsg needs to be global, or you can get
a SDYNIMPORT symbol that has .Hide == true) so handling these cases
in genasmsym makes everything much more regular.

Even ignoring shared libraries, I think this is a bit cleaner.

Change-Id: If5beb093a261e79f4496183226e1765ee7aa6717
Reviewed-on: https://go-review.googlesource.com/8230
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>