cmd/go: permit $AR to include options

Handle the AR environment variable, used by gccgo,
the same way we handle the CC environment variable.

This ports https://go.dev/cl/526275 to the gofrontend repo.

This is needed for gccgo testing because the top-level GCC Makefile
now passes a --plugin option to ar if it supports one.

Change-Id: I54cd6b6d5b1a757ea112c159414b2f24fa0fb25a
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/526295
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
diff --git a/libgo/go/cmd/go/internal/work/gccgo.go b/libgo/go/cmd/go/internal/work/gccgo.go
index c1026c7..3e07f87 100644
--- a/libgo/go/cmd/go/internal/work/gccgo.go
+++ b/libgo/go/cmd/go/internal/work/gccgo.go
@@ -45,12 +45,8 @@
 	return GccgoBin
 }
 
-func (gccgoToolchain) ar() string {
-	ar := cfg.Getenv("AR")
-	if ar == "" {
-		ar = "ar"
-	}
-	return ar
+func (gccgoToolchain) ar() []string {
+	return envList("AR", "ar")
 }
 
 func checkGccgoBin() {