[dev.link] cmd/link: add loader method to sort sub-symbols of outer symbol

[dev.link] cmd/link: add loader method to sort sub-symbols of outer symbol

Add a new loader method SymSortSub that sorts the sub-symbols
of a given outer symbol (designed to be compatible with the
existing sym.Symbol method).

Change-Id: Icd6627b2e6d04524d657e712cfd39fda0e0e080b
Reviewed-on: https://go-review.googlesource.com/c/go/+/211297
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jeremy Faller <jeremy@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>