[dev.link] cmd/link/internal/loader: copy Dynid when migrating sym attrs
[dev.link] cmd/link/internal/loader: copy Dynid when migrating sym attrs

Add support for migrating over the symbol Dynid property
when converting loader.Sym symbols to sym.Symbol.

Change-Id: Icc3b91b4adcae6f2ede7d915bb674cc206025217
Reviewed-on: https://go-review.googlesource.com/c/go/+/226360
Run-TryBot: Than McIntosh <4b2922593166fc595a9b1a408f34f5d3817fe9d2@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>

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