cmd/link: convert symbol Add* functions to methods

cmd/link: convert symbol Add* functions to methods

Also reduce the passed context from *Link to *sys.Arch, so fewer
data dependencies need to be wired through all the code dealing
with symbols.

For #22095

Change-Id: I50969405d6562c5152bd1a3c443b72413e9b70bc
Reviewed-on: https://go-review.googlesource.com/67313
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>