cmd/go: fix buglet in alternate gccgo debug_modinfo recipe

Fix bug in previous CL 171768 -- with Go 1.13 the proper entry point
to call is runtime.setmodinfo, not runtime..z2fdebug.setmodinfo (this
changed when we moved from 1.12). [ Unclear why trybots and runs of
all.bash didn't catch this, but hand testing made it apparent. ]

Updates #30344.

Change-Id: I91f47bd0c279ad2d84875051be582818b13735b6
Reviewed-on: https://go-review.googlesource.com/c/go/+/196237


Run-TryBot: Than McIntosh <thanm@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>