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/debug.go

@@ -57,3 +57,8 @@ func NumCgoCall() int64 {
 func NumGoroutine() int {
 	return int(gcount())
 }
+
+//go:linkname debug_modinfo runtime/debug.modinfo
+func debug_modinfo() string {
+	return modinfo
+}