[dev.link] cmd/link: add Errorf method on context taking new sym
[dev.link] cmd/link: add Errorf method on context taking new sym

Add a Link method that takes a loader sym index instead
of a sym.Symbol.

Change-Id: If59e246cb1adc9066080f019be135387fc1b2fcd
Reviewed-on: https://go-review.googlesource.com/c/go/+/208228
Reviewed-by: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>

nawawi/go,cloudflare/go,golang/go,akutz/go,nawawi/go,cloudflare/go,nawawi/go,golang/go,nawawi/go,akutz/go,golang/go,CAFxX/go,akutz/go,golang/go,CAFxX/go,golang/go,cloudflare/go,cloudflare/go,CAFxX/go,akutz/go,nawawi/go,akutz/go,CAFxX/go,golang/go,nawawi/go,CAFxX/go,cloudflare/go,CAFxX/go,nawawi/go,golang/go,nawawi/go,cloudflare/go,CAFxX/go,akutz/go,CAFxX/go,golang/go,cloudflare/go,cloudflare/go,akutz/go,akutz/go