runtime: uncomment check for gchelper on g0 stack

Now that systemstack changes to the g0 stack, this check passes.
    
    Reviewed-on: https://go-review.googlesource.com/46460

From-SVN: r249578

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-c5aa42107d116c9121de447d214297c88dd9f5fa
+385efb8947af70b8425c833a1ab68ba5f357dfae
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/runtime/mgc.go

@@ -1923,10 +1923,9 @@ func gchelperstart() {
 	if _g_.m.helpgc < 0 || _g_.m.helpgc >= _MaxGcproc {
 		throw("gchelperstart: bad m->helpgc")
 	}
-	// For gccgo we run gchelper on the normal g stack.
-	// if _g_ != _g_.m.g0 {
-	// 	throw("gchelper not running on g0 stack")
-	// }
+	if _g_ != _g_.m.g0 {
+		throw("gchelper not running on g0 stack")
+	}
 }
 
 // itoaDiv formats val/(10**dec) into buf.