cmd/cgo: detect misuse of generated _cgo_export.h

Fixes #9742.

Change-Id: Ifedf7ff9465bc49534b708d414c8e435ee9ce6cd
Reviewed-on: https://go-review.googlesource.com/3970Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/cgo/out.go

@@ -636,7 +636,7 @@ func (p *Package) writeExports(fgo2, fm io.Writer) {
 	fgcc := creat(*objDir + "_cgo_export.c")
 	fgcch := creat(*objDir + "_cgo_export.h")
 
-	fmt.Fprintf(fgcch, "/* Created by cgo - DO NOT EDIT. */\n")
+	fmt.Fprintf(fgcch, "/* Created by cgo - DO NOT EDIT. */\n/*  This file is arch-specific.  */\n")
 	fmt.Fprintf(fgcch, "%s\n", p.Preamble)
 	fmt.Fprintf(fgcch, "%s\n", p.gccExportHeaderProlog())
 
@@ -1310,6 +1310,10 @@ typedef double GoFloat64;
 typedef __complex float GoComplex64;
 typedef __complex double GoComplex128;
 
+// static assertion to make sure the file is being used on architecture
+// at least with matching size of GoInt.
+typedef char _check_for_GOINTBITS_bit_pointer_matching_GoInt[sizeof(void*)==GOINTBITS/8 ? 1:-1];
+
 typedef struct { char *p; GoInt n; } GoString;
 typedef void *GoMap;
 typedef void *GoChan;