cmd/link/internal/ld: fix TestArchiveBuildInvokeWithExec

TestArchiveBuildInvokeWithExec is failing on darwin due to
duplicated symbols, because the C definition (int fortytwo;) is
copied to two generated cgo sources. In fact, this test is about
building c-archive, but doesn't need to import "C". Removed the
"C" import.

Change-Id: I3a17546e01272a7ae37e6417791ab949fb44597e
Reviewed-on: https://go-review.googlesource.com/c/go/+/205278


Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>