cmd/internal/obj: replace AGLOBL with (*Link).Globl

cmd/internal/obj: replace AGLOBL with (*Link).Globl

Replace the AGLOBL pseudo-op with a method to directly register an
LSym as a global. Similar to how we previously already replaced the
ADATA pseudo-op with directly writing out data bytes.

Passes toolstash -cmp.

Change-Id: I3631af0a2ab5798152d0c26b833dc309dbec5772
Reviewed-on: https://go-review.googlesource.com/29366
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Dave Cheney <dave@cheney.net>
TryBot-Result: Gobot Gobot <gobot@golang.org>