test: update tinyfin test

* use new(int32) to be pedantic about documented SetFinalizer rules:
  "The argument x must be a pointer to an object allocated by calling
  new or by taking the address of a composite literal"

* remove the amd64-only restriction. The GC is fully precise everywhere
  now, even on 32-bit. (keep the gccgo restriction, though)

* remove a data race (perhaps the actual bug) and use atomic.LoadInt32
  for the final check. The race detector is now happy, too.

Updates #13100

Change-Id: I8d05c0ac4f046af9ba05701ad709c57984b34893
Reviewed-on: https://go-review.googlesource.com/16535


Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>