[dev.link] cmd/internal/goobj2, cmd/link: add accessors for field of FuncInfo

[dev.link] cmd/internal/goobj2, cmd/link: add accessors for field of FuncInfo

Add accessors for fields of FuncInfo, so we don't have to read
the whole FuncInfo.

TODO: explore/experiment with an alternative idea -- splitting
FuncInfo to separate Aux symbols.

Change-Id: Ie4bc2613fd76d08fc63fd86956802920da63dd2f
Reviewed-on: https://go-review.googlesource.com/c/go/+/220979
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>