test: add testcase for gccgo compiler buglet

New test containing code that caused a gccgo compiler failure.

Updates #34503.

Change-Id: Id895a1e1249062b7fb147e54bcaa657e774ed0d9
Reviewed-on: https://go-review.googlesource.com/c/go/+/197217
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

test/fixedbugs/issue34503.dir/a.go

+// 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 a
+
+import "unsafe"
+
+type HookFunc func(x uint64)
+
+var HookV unsafe.Pointer
+
+func Hook(x uint64) {
+	(*(*HookFunc)(HookV))(x)
+}

test/fixedbugs/issue34503.dir/b.go

+// 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 b
+
+import "a"
+
+func Bfunc() {
+	a.Hook(101)
+}

test/fixedbugs/issue34503.go

+// compiledir
+
+// 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.
+
+// Issue 34503: gccgo compiler error importing inlinable function
+
+package ignored