test: add test gcc78763.go that caused a gccgo compiler crash

Change-Id: I0d990d494ee8eebfc804b526f5ced71944572b4d
Reviewed-on: https://go-review.googlesource.com/34491
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

test/fixedbugs/gcc78763.go

+// compile
+
+// Copyright 2016 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.
+
+// The gccgo compiler crashed while compiling this code.
+// https://gcc.gnu.org/PR78763.
+
+package p
+
+import "unsafe"
+
+func F() int {
+	if unsafe.Sizeof(0) == 8 {
+		return 8
+	}
+	return 0
+}