[dev.link] cmd/link/internal/loader: add Loader plt/got access methods
[dev.link] cmd/link/internal/loader: add Loader plt/got access methods

Add accessor methods to get at the symbol {plt,got} value for
PE symbols. Fix a bug in the loaders SetPlt/SetGot methods.

Change-Id: I975bd6b86122622b206487c8798f8290ecd25a57
Reviewed-on: https://go-review.googlesource.com/c/go/+/225199
Run-TryBot: Than McIntosh <4b2922593166fc595a9b1a408f34f5d3817fe9d2@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>

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