[dev.link] cmd/internal/goobj2, cmd/link: experiment new aux symbol accessors

[dev.link] cmd/internal/goobj2, cmd/link: experiment new aux symbol accessors

Following the previous CLs, do the same for aux symbols. This has
some small speedup:

(linking cmd/compile)
Dostkcheck    41.0ms ± 1%    38.6ms ± 1%    -6.00%  (p=0.008 n=5+5)

Change-Id: Id62b2fc9e4ef1be92e60e4c03faec0a953eee94e
Reviewed-on: https://go-review.googlesource.com/c/go/+/222303
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>