make.bash: more robustly detect gold 2.20

On recent Debian systems the gold 2.20 check triggers though
Debian doesn't have version 2.20 but rather has:

        GNU gold (GNU Binutils for Debian 2.21.52.20110606) 1.11
                                                ^^^^

R=rsc, iant
CC=golang-dev
https://golang.org/cl/5252055
1 file changed