[dev.link] cmd/link: use Extname in doxcoff

[dev.link] cmd/link: use Extname in doxcoff

On AIX, when external linking, we need to change the function
names to start with a dot and make function descriptors with the
names without the dot. Currently this is done through symbol
renaming, which is not friendly for switching to the loader.

In this CL we use symbol's external name for this. This allows us
to get rid of symbol renaming.

Change-Id: If72602d17e96f0339fdac2e2321f1edfb292b5f9
Reviewed-on: https://go-review.googlesource.com/c/go/+/224940
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>