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

Fixes #34968.

Change-Id: I538d653fab6cf7cf9b9b7022a1c2d4ae6ee497b6
Reviewed-on: https://go-review.googlesource.com/c/go/+/201823
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -75,7 +75,7 @@ func pragmaValue(verb string) syntax.Pragma {
 	case "go:yeswritebarrierrec":
 		return Yeswritebarrierrec
 	case "go:cgo_unsafe_args":
-		return CgoUnsafeArgs
+		return CgoUnsafeArgs | NoCheckPtr // implies NoCheckPtr (see #34968)
 	case "go:uintptrescapes":
 		// For the next function declared in the file
 		// any uintptr arguments may be pointer values

test/fixedbugs/issue34968.go

@@ -0,0 +1,15 @@
+// +build cgo
+// run -gcflags=all=-d=checkptr
+
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+// #include <stdlib.h>
+import "C"
+
+func main() {
+	C.malloc(100)
+}