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

This reverts CL 186417.

Reason for revert: Broke darwin (10_14), linux (ppc), aix (ppc)

Updates #33139

Change-Id: I8bf3c817a96a0e57e45754a097cea7062b2fcdfd
Reviewed-on: https://go-review.googlesource.com/c/go/+/198177


Run-TryBot: Andrew Bonventre <andybons@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>