[dev.link] cmd/compile: refactor aux handling in newobj sym traversal

[dev.link] cmd/compile: refactor aux handling in newobj sym traversal

Generalize symbol traversal code for aux symbols to allow for client
control over whether the walk incldues symbols referenced by
relocations on visited aux syms. This is not needed just yet but will
be required in order to support anonymous aux syms that have
relocations.

Change-Id: I898c1f398213c8d9d777dd3c40524a013b25e348
Reviewed-on: https://go-review.googlesource.com/c/go/+/223668
Reviewed-by: Cherry Zhang <cherryyz@google.com>