cmd/compile: disable checkptr for //go:nosplit functions

Make go test -a -short -gcflags=all=-d=checkptr passes on darwin.

Update #34972

Change-Id: I71cf14ec1faccd4837713aa30c90ed665899b908
Reviewed-on: https://go-review.googlesource.com/c/go/+/202158
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -61,7 +61,7 @@ func pragmaValue(verb string) syntax.Pragma {
 	case "go:norace":
 		return Norace
 	case "go:nosplit":
-		return Nosplit
+		return Nosplit | NoCheckPtr // implies NoCheckPtr (see #34972)
 	case "go:noinline":
 		return Noinline
 	case "go:nocheckptr":