cmd/go: move runtime/debug.modinfo to runtime.modinfo

It is easier to ensure that the symbol is always present
if we move it to package runtime. Avoids init-time work.
Also moves it next to buildVersion, the other similar symbol.

Setting up for "go version <binary>".

For #31624.

Change-Id: I943724469ce6992153e701257eb6f12da88c8e4e
Reviewed-on: https://go-review.googlesource.com/c/go/+/173341
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/proc.go

@@ -13,6 +13,9 @@ import (
 
 var buildVersion = sys.TheVersion
 
+// set using cmd/go/internal/modload.ModInfoProg
+var modinfo string
+
 // Goroutine scheduler
 // The scheduler's job is to distribute ready-to-run goroutines over worker threads.
 //
@@ -577,6 +580,11 @@ func schedinit() {
 		// to ensure runtime·buildVersion is kept in the resulting binary.
 		buildVersion = "unknown"
 	}
+	if len(modinfo) == 1 {
+		// Condition should never trigger. This code just serves
+		// to ensure runtime·modinfo is kept in the resulting binary.
+		modinfo = ""
+	}
 }
 
 func dumpgstatus(gp *g) {