runtime: add padding to FFI type of struct ending with zero-sized field

CL 157557 changes the compiler to add one byte padding to
    non-empty struct ending with a zero-sized field. Add the same
    padding to the FFI type, so reflect.Call works.
    
    This fixes test/fixedbugs/issue26335.go in the main repo.
    
    Reviewed-on: https://go-review.googlesource.com/c/158018

From-SVN: r267956

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-87005025fcd0d7e7908b3aae7062b52cb80eb0f3
+9a79c333e896ea49f6a708d459148074d29a2af6
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/runtime/ffi.go

@@ -227,6 +227,7 @@ func structToFFI(typ *structtype) *__ffi_type {
 
 	fields := make([]*__ffi_type, 0, c+1)
 	checkPad := false
+	lastzero := false
 	for i, v := range typ.fields {
 		// Skip zero-sized fields; they confuse libffi,
 		// and there is no value to pass in any case.
@@ -235,8 +236,10 @@ func structToFFI(typ *structtype) *__ffi_type {
 		// next field.
 		if v.typ.size == 0 {
 			checkPad = true
+			lastzero = true
 			continue
 		}
+		lastzero = false
 
 		if checkPad {
 			off := uintptr(0)
@@ -257,6 +260,13 @@ func structToFFI(typ *structtype) *__ffi_type {
 		fields = append(fields, typeToFFI(v.typ))
 	}
 
+	if lastzero {
+		// The compiler adds one byte padding to non-empty struct ending
+		// with a zero-sized field (types.cc:get_backend_struct_fields).
+		// Add this padding to the FFI type.
+		fields = append(fields, ffi_type_uint8())
+	}
+
 	fields = append(fields, nil)
 
 	return &__ffi_type{