cmd/compile: make sure linkname'd symbol is non-package

When a variable symbol is both imported (possibly through
inlining) and linkname'd, make sure its LSym is marked as
non-package for symbol indexing in the object file, so it is
resolved by name and dedup'd with the original definition.

Fixes #42401.

Change-Id: I8e90c0418c6f46a048945c5fdc06c022b77ed68d
Reviewed-on: https://go-review.googlesource.com/c/go/+/268178


Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>