runtime: improve systemstack-on-Go stack message

We reused the old C stack check mechanism for the implementation of
//go:systemstack, so when we execute a //go:systemstack function on a
user stack, the system fails by calling morestackc. However,
morestackc's message still talks about "executing C code".

Fix morestackc's message to reflect its modern usage.

Change-Id: I7e70e7980eab761c0520f675d3ce89486496030f
Reviewed-on: https://go-review.googlesource.com/38572
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/stack.go

@@ -1188,6 +1188,6 @@ func freeStackSpans() {
 //go:nosplit
 func morestackc() {
 	systemstack(func() {
-		throw("attempt to execute C code on Go stack")
+		throw("attempt to execute system stack code on user stack")
 	})
 }