cmd/link: add new linker testpoint for "ld -r" host object

This adds a new test that builds a small Go program with linked
against a *.syso file that is the result of an "ld -r" link. The
sysobj in question has multiple static symbols in the same section
with the same name, which triggered a bug in the loader in -newobj
mode.

Updates #35779.

Change-Id: Ibe1a75662dc1d49c4347279e55646ee65a81508e
Reviewed-on: https://go-review.googlesource.com/c/go/+/208478


Reviewed-by: Cherry Zhang <cherryyz@google.com>