cmd/compile/internal/gc: allow //go:systemstack only in runtime

Fixes #12454.

Change-Id: I6406b0119bc4d6c3d1e6f1896b588b7d101448a3
Reviewed-on: https://go-review.googlesource.com/14274
Run-TryBot: Minux Ma <minux@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/compile/internal/gc/lex.go

@@ -1623,6 +1623,9 @@ func getlinepragma() int {
 		}
 
 		if verb == "go:systemstack" {
+			if compiling_runtime == 0 {
+				Yyerror("//go:systemstack only allowed in runtime")
+			}
 			systemstack = true
 			return c
 		}