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