cmd/link: load symbols from .syso in external link mode

Fix linking with a package having a .syso file in external link mode,
that would otherwise cause an error before executing the external
linker because it can't find symbols that are exported in the said
.syso file.

Fixes #33139

Change-Id: Id3ee737fba1c6f1e37910593dfedf9c84486d398
Reviewed-on: https://go-review.googlesource.com/c/go/+/186417


Reviewed-by: Ian Lance Taylor <iant@golang.org>