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.

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/526295

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-d04b024021bb7dbaa434a6d902bd12beb08e315f
+53cdfab53ce79d8dfc1df01a696d4ffc43f17c3d
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/cmd/go/internal/work/gccgo.go

@@ -45,12 +45,8 @@ func (gccgoToolchain) linker() string {
 	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() {