cmd/link/internal/amd64: -buildmode=c-archive forces external link mode

At some point this code should probably move to cmd/link/internal/ld,
but at least for now just handle c-archive like c-shared.

Change-Id: Ic17656529cb0fe189a37f15e670350ab13bb5276
Reviewed-on: https://go-review.googlesource.com/10385Reviewed-by: Andrew Gerrand <adg@golang.org>

src/cmd/link/internal/amd64/obj.go

@@ -90,7 +90,7 @@ func archinit() {
 		ld.Linkmode = ld.LinkInternal
 	}
 
-	if ld.Buildmode == ld.BuildmodeCShared || ld.DynlinkingGo() {
+	if ld.Buildmode == ld.BuildmodeCArchive || ld.Buildmode == ld.BuildmodeCShared || ld.DynlinkingGo() {
 		ld.Linkmode = ld.LinkExternal
 	}