cgo: use correct frame size for 0 arguments

Passing a frame size of 1 was causing the cgo callback
to push 1 byte of arguments onto the stack, making
the stack pointer misaligned, which had the effect of
hiding all the pointers on the stack from the garbage
collector.

SWIG only wraps calls to C++ virtual methods, so it
always has at least 1 argument, so SWIG does not need
to be fixed too.

Fixes #1328.

R=iant
CC=golang-dev
https://golang.org/cl/4261046

src/cmd/cgo/out.go

@@ -180,7 +180,6 @@ func (p *Package) structType(n *Name) (string, int64) {
 	}
 	if off == 0 {
 		fmt.Fprintf(&buf, "\t\tchar unused;\n") // avoid empty struct
-		off++
 	}
 	fmt.Fprintf(&buf, "\t}")
 	return buf.String(), off
@@ -225,6 +224,9 @@ func (p *Package) writeDefsFunc(fc, fgo2 *os.File, n *Name) {
 	fmt.Fprintf(fc, "void _cgo%s%s(void*);\n", cPrefix, n.Mangle)
 	fmt.Fprintf(fc, "\n")
 	fmt.Fprintf(fc, "void\n")
+	if argSize == 0 {
+		argSize++
+	}
 	fmt.Fprintf(fc, "·%s(struct{uint8 x[%d];}p)\n", n.Mangle, argSize)
 	fmt.Fprintf(fc, "{\n")
 	fmt.Fprintf(fc, "\truntime·cgocall(_cgo%s%s, &p);\n", cPrefix, n.Mangle)
@@ -392,7 +394,6 @@ func (p *Package) writeExports(fgo2, fc, fm *os.File) {
 		}
 		if ctype == "struct {\n" {
 			ctype += "\t\tchar unused;\n" // avoid empty struct
-			off++
 		}
 		ctype += "\t}"
 
@@ -444,7 +445,7 @@ func (p *Package) writeExports(fgo2, fc, fm *os.File) {
 			func(i int, atype ast.Expr) {
 				fmt.Fprintf(fgcc, "\ta.p%d = p%d;\n", i, i)
 			})
-		fmt.Fprintf(fgcc, "\tcrosscall2(_cgoexp%s_%s, &a, (int) sizeof a);\n", cPrefix, exp.ExpName)
+		fmt.Fprintf(fgcc, "\tcrosscall2(_cgoexp%s_%s, &a, %d);\n", cPrefix, exp.ExpName, off)
 		if gccResult != "void" {
 			if len(fntype.Results.List) == 1 && len(fntype.Results.List[0].Names) <= 1 {
 				fmt.Fprintf(fgcc, "\treturn a.r0;\n")