misc/cgo/testso, misc/cgo/testsovar: fix for Windows

misc/cgo/testso, misc/cgo/testsovar: fix for Windows

At least in mingw-clang it is not permitted to just name a .dll
on the command line. You must name the corresponding import
library instead, even though the dll is used when the executable
is run.

This fixes misc/cgo/testso and misc/cgo/testsovar on windows/arm64.

Change-Id: I516b6ccba2fe3a9ee2c01e710a71850c4df8522f
Reviewed-on: https://go-review.googlesource.com/c/go/+/312046
Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>