[dev.link] cmd/link: process cgo_import_static before host object loading

[dev.link] cmd/link: process cgo_import_static before host object loading

In internal linking mode, we need to process cgo_import_static
directives before loading host objects, because the directive is
to tell the host object loader how to deal with imported symbols.

This should fix linking with old object files. I think there
needs some similar logic for new object files, but I'll leave
that for later.

Change-Id: Icaa286de626ea1876086dbdd015047084c92caf9
Reviewed-on: https://go-review.googlesource.com/c/go/+/204697
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>