cmd/go: fix buglet in alternate gccgo debug_modinfo recipe
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 <4b2922593166fc595a9b1a408f34f5d3817fe9d2@google.com>
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

cloudflare/go,akutz/go,cloudflare/go,nawawi/go,nawawi/go,golang/go,golang/go,CAFxX/go,CAFxX/go,golang/go,golang/go,akutz/go,golang/go,cloudflare/go,nawawi/go,cloudflare/go,akutz/go,nawawi/go,akutz/go,CAFxX/go,akutz/go,nawawi/go,cloudflare/go,golang/go,akutz/go,nawawi/go,CAFxX/go,akutz/go,CAFxX/go,cloudflare/go,CAFxX/go,cloudflare/go,cloudflare/go,golang/go,akutz/go,golang/go,nawawi/go,CAFxX/go,CAFxX/go,nawawi/go