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>

src/cmd/go/internal/modload/build.go

@@ -274,7 +274,7 @@ var __debug_modinfo__ = %q
 	} else {
 		return []byte(fmt.Sprintf(`package main
 import _ "unsafe"
-//go:linkname __set_debug_modinfo__ runtime..z2fdebug.setmodinfo
+//go:linkname __set_debug_modinfo__ runtime.setmodinfo
 func __set_debug_modinfo__(string)
 func init() { __set_debug_modinfo__(%q) }
 	`, string(infoStart)+info+string(infoEnd)))